American National Election Studies (ANES) -- The ANES series is one of the premier sources for data on voting behavior and political attitudes in the post-WWII United States. Upon registering (which is free), users can download data and SAS/SPSS/Stata program files and documentation for any of the various NES studies. The SDA Archive at UC-Berkeley also allows users to access recent versions of the ANES and extract particular variables from them for those who have specific questions in which they are interested.
American National Election Studies ANES Guide to Public Opinion and Electoral Behavior -- The ANES Guide is an excellent source for data on trends in voting behavior for the U.S. since World War II.
Adam Carr's Elections Archive -- Excellent link for electoral results around the world, with a special section on Australia. Results are for the most recent elections and are typically broken down into regions or the equivalent electoral constituency.
Comparative Study of Electoral Systems -- This site contains data and documentation for a cross-national collection of election studies. The CSES combines microdata on respondents with data at different levels of geography (i.e. data on respondents' electoral districts and on the political institutions of their countries) and focuses on themes such as how macro-level variables such as electoral systems affect political attitudes.
Constituency-Level Elections Archive (CLEA) -- The CLEA is a project hosted by the University of Michigan's Center for Political Studies and is "a repository of detailed results - including votes received by each candidate/party, total votes cast, number of eligible voters, and seat figures where available - at a constituency level for the lower house legislative elections that have been conducted around the world."
CQ Voting and Elections Collection -- The CQ Voting and Elections Collection is part of the CQ Electronic Library that is in Databases at Emory. It contains a variety of data for federal elections and gubernatorial elections at the national, state, and county district levels.
Current Population Survey Voting and Registration Data -- The Current Population Survey gathers data on voting and voter registration in November of congressional and presidential election years. The Census Bureau provides summary tables from the CPS. Microdata files from the Voting and Registration Suppelments are available from an FTP site, from the National Bureau of Economic Research's CPS site or from IPUMS-CPS.
Democratic Electoral Systems Around the World, 1946-2016 -- This collection compiled by Matthew Golder provides data for "some of the more important electoral institutions used in all legislative and presidential elections during democratic periods in 199 countries between 1946 (or independence) and 2000 (...) The dataset covers a wide range of institutional features including regime type, the electoral formula, the average and median district magnitude, the number of constituencies and upper tier seats, assembly size etc." The data were recently updated to provide coverage up through 2016. The data are also available via Golder's Dataverse.
Election Data Archive -- Harvard's Election Data Archive provides precinct-level data for results from federal elections for individual states, for 2000 onwards varying by state. Many of the data on results are accompanied by shapefiles for boundaries of electoral precints, for use with GIS software.
Election Resources on the Internet (Manuel Alvarez-Rivera) -- Good resource for links to electoral data and analysis, organized by country.
European Election Database (EED) -- The EED, which is hosted by Norwegian Social Science Data Services, "publishes regional election results according to the Nomenclature of Territorial Units for Statistics (NUTS), level 1 to 3" for a selection of 35 European countries for 1990 onwards. Data are available for parliamentary, presidential, and European Parliament elections.
European Election Studies (EES) -- To quote the website, the EES Data "are about electoral participation and voting behaviour in European Parliament elections" as well as perceptions about the EU itself. The EES data consist of studies of voters, of elites, of media content, and of party manifestos. Much of the data from this project are available via GESIS.
Global Elections Database -- The Global Elections Database is a project undertaken by Professor Dawn Brancati at Columbia University and is a compilation of district-level election results broken by down individual political parties. The data cover national elections and some sub-national elections as well.
Institute for Democracy and Electoral Assistance (IDEA) -- IDEA provides multiple tools and databases on electoral systems, including election results, voter turnout, electoral systems, and electoral quotas.
Manifesto Project Database -- The Manifesto Project Database "provides the scientific community with parties’ policy positions derived from a content analysis of parties’ electoral manifestos. It covers over 1000 parties from 1945 until today in over 50 countries on five continents." The geographic coverage includes Western Europe, Central/Eastern Europe, the Americas, and other selected countries. The database provides access to both the text of party manifestos and data derived from those manifestos that code parties on issues such as welfare spending, state intervention in markets, globalization, gender equality, and European integration. There are also tools for using the data in R or in Stata.
MIT Election Data Science Lab (MEDSL) -- The MEDSL provides precinct-level data for recent federal, state, and local elections. Data on "election performance index" scores for the quality of administration of elections in individual states are also available.
National Elections Across Democracy and Autocracy (NELDA) -- The NELDA collection at Yale provide election-event-level data on the outcomes and characteristics of individual elections for national legislative and executive elections between 1960 and 2012. The data cover topics such as the presence of election monitors, measures of electoral manipulation, and whether there was any post-election violence.
OpenSecrets -- OpenSecrets is a watchdog group with an extensive clearinghouse of data on campaign contributions and lobbying. In addition to its main site, OpenSecrets also has an Open Data Initiative in which users can register to download bulk amounts of OpenSecrets' data for additional analysis.
ParlGov - Parliaments and Governments Database -- "ParlGov is a data infrastructure for political science and contains information for all EU and most OECD democracies (37 countries). The database combines approximately 1500 parties, 920 elections (8400 results), and 1400 cabinets (3500 parties). "
Parties and Elections in Europe -- Parties and Elections in Europe provides election results for national elections and for sub-national elections for selected states, for most of the post-WWII period. The data cover both Western and Central/Eastern Europe.
Party Government Data Set (PGDS) -- The Party Government Data Set, hosted by J.J Woldendorp at the University of Amsterdam, "covers 39 parliamentary democracies from 1945, or the year these countries became a parliamentary democracy (again), through 2008." The geographic focus is on countries in Western and Eastern Europe. The data cover topics such as reason for government termination, ideological orientation of government, names of parties in government, and size of governing coaliation (e.g. minority governments).
Policy Agendas Project -- UT-Austin's Policy Agendas Project is an ambitious undertaking that strives to "[collect and organize] data from various archived sources to trace changes in the national policy agenda and public policy outcomes since the Second World War." The data collections cover items such as Presidential State of the Union Addresses, roll-call votes and committee hearings in Congress, federal spending, and the priorities and "moods" of the public.
Political Database of the Americas -- Holds numerous election results from most North, Central and South American countries. Researchers should note, however, that data is not always thorough for each country and can differ widely in terms of coverage and detail.
Roper Center for Public Opinion Research -- As part of its archive of public-opinion data, the Roper Center has a large collection of national and state exit polls for elections and primaries back to the 1970's. Roper also provides presidential approval ratings for FDR onwards.
VoteView -- VoteView, which is a project of Dr. Keith Poole, provides freely-available Congressional data for roll-call votes, party polarization, and ideology scores. See https://legacy.voteview.com/ for data from earlier iterations of the project.