Academic Search Complete is a multidisciplinary search engine that indexes magazine and journal articles. Both its strength and its weakness for philosophers is that one may find resources that are perhaps relevant to one's search, but from other fields. A good place to start, especially if your research is more interdisciplinary.
JSTOR (short for Journal Storage), is a digital library containing a collection of articles, books, and other sources. From their website: "JSTOR currently offers more than 10 million academic journal articles, 50,000 books, and 2 million primary source documents in 75 disciplines." Note that JSTOR generally does not have the most recent articles from a given journal; for those, Project Muse is a better option.
JSTOR also recently launched the beta version of their "Text Analyzer," a tool that allows you to upload a document (article, book, etc.), which will generate a list of search terms and suggest other relevant material. You can try out the JSTOR Text Analyzer here.
Unlike Academic Search Complete, JSTOR, or Project Muse, Philosopher's Index focuses exclusively on philosophy books and journals. Their journal collection includes both general journals about philosophy in a broad sense, and more specialized journals focusing on specific figures, issues, traditions, etc. If you are looking to focus in on a specifically philosophical topic without needing to sort through articles in, e.g., political science, psychology, or social science journals, Philosopher's Index is a great place to look.
The Philosophy Documentation Center (PDC) contains a collection of numerous philosophy journals, both historical and contemporary, as well as a number of philosophy books. All journals and books are fully accessible online. As there are a large number of journals, not all of which have helpful or revealing titles as to their contents, the "By Category" page can be helpful in narrowing your search (E-Collection -> By Category).
PhilPapers is a good database for all things philosophy, covering not only journals and books, but also things like open access archives and personal pages maintained by academics. In addition to their database, PhilPapers also offers helpful category/subcategory pages, journal pages, and bibliographies. One unfortunate downside of PhilPapers is that because its emphasis is on information, not access, you may have to get access to particular items through other means, such as JSTOR, Philosopher's Index, or DiscoverE.
Oxford Bibliographies contain extensive entries on both topics and figures in the history of philosophy. Broken down into subsections, each page has an annotated bibliography of works pertaining to that entry, both in terms of primary and secondary literature. These can include historical context, philosophical associations with contemporaries, disciples, major subtopics, overview-type works, and more.