The Best Sources: Secondary and Primary
A responsible paper will make use of the best available sources of information. Do not accept information presented at third hand. Seek secondary sources that reflect directly on the primary evidence. Use scholars who work in a dialogue with their peers. Use primary sources that provide the relevant raw evidence for the study wherever possible.
Google BooksGoogle Book Search or just plain "Google Books" is the best place to conduct research if you are looking for any book that is in the public domain (usually, this means works or translations that are about 80 years of age and older). Don't forget to set the pull-down display results bar to "Full Text Only." Also, be prepared to use your most skillful search techniques; Google's indexing and cataloguing is a mess. If you are looking for readily accessible English translations of primary source literature (albeit not always based on the best critical texts), you will find many great works available here. By the way: what you can't find on Google books look for on Archive.org.
WorldCatWorldCat is totally awesome. The database contains entries on full catalogs of thousands of libraries. There is a Public WorldCat and a Subscription WorldCat (the latter link is for NCLIVE customers only). Use WorldCat to invesitage the publication history of books, to verify publication information, and otherwise make your bibliography sound.
Gale Virtual Reference LibraryGale Virtual Reference Library (this link is designed for Mars Hill College students only) has a huge selection of very authoritative academic encyclopedias. The search features let you see every article containing the search terms you employ—you score dozens of articles from a single search. The encyclopedias offer authoritative introductory and basic information (like dates and chronologies, biographical data, bibliographic entries, subject introductions, etc.) in thousands of sujects. The chief weakness of many student papers is a failure do their basic research in the encyclopedias before setting out into the secondary and primary literature.
JSTORJSTOR (this link is for ACA and MHC students only) is by far the coolest of the places to find secondary, peer-reviewed literature. The huge database at JSTOR contains the full text of thousands of volumes of the leading academic journals. An outstanding collection of religious, philosophical, classical, Biblical, and theological studies journals is archived at JSTOR. The articles in this database are diverse, and span almost 200 years of scholarship: from the early 19th to the early 21st century.
Online Libraries of Ancient Primary SourcesMany different collections of primary (and secondary) sources can be found online. For students of world religion and mythology, the collection of public domain texts at SacredTexts.com can prove incredibly useful.
For students of Christianity, the collection at Christian Classics Ethereal Library is a wonderful aid. For students of early Christian literature, a great library can be found at Gnosis.org. The library of Ancient Near Eastern texts found at ETANA.org (Electroic Tools and Ancient Near Eastern Archives) is absolutely fabulous. The Electronic Text Corpus of Sumerian Literature from the Oxford Oriental Institute is very good on ancient Akkadian and other Mesopotamian texts. For ancient texts in general, one can use The Ancient History Sourcebook at Fordham and The Perseus Digital Library at Tufts. For Greco-Roman texts you can use Lacus Curtius at University of Chicago. The Internet Classics Archive at MIT contains many important works. The abriged online version of the Thesaurus Linguae Grecae contains the full Greek text of a number of important authors.
Basically every ancient text which existed in a printed edition prior to 1929 is available for free somewhere online. You just have to figure out how to search for it. The Latin titles used for ancient works in the early 19th century are sometimes the key to finding these antique editions. Be savvy in your searching, and you will be amazed at what you turn up.