This is from my amazing friend Yair.
Disclaimer: I'm by no means a certified rabbi or scholar. I simply like to compile useful information from the web into one lump.
That being said, many of us find ourselves at various times during our lives in dire need of a sefer, but may not/cannot/are too lazy to find an actual hard copy of it. Luckily, the Internet has more than we think it does. The following is just a sample (I'm sure) of the amount available online. Don't fret if, at first, it seems too much to handle. Just play around with it a bit and hopefully everything should be fine. All of the links (and most if not all of the links within each site) are working, as of 10/29/07.
Practically Everything (Yair's personal favorite)
Halacha Brura and Birur Halacha Institute
All Your Basic Hebrew Text Needs
M'Ager Sifrot Kodesh
Soncino Talmud (most tractates)
Internet Sacred Text Archive
Seforim Online (Lots of philosophical/ scientific stuff)
Hebrew Books (specializes in 20th century American journals)
Teach it to Me (downloadable sefarim)
Tsel Harim (Free Torah library)
JNUL Library (You may need to download the DjVu plug-in to view certain files)
Bar-Ilan Responsa Database (You might need a subscription form, in which case log in from one of YU's campuses)
Comprehensive Aramaic Lexicon
Database Query to Semitic Etymology
Chronological Timeline of Kings
Ancient Maps of Jerusalem
More than any Kabbalah Sefer you can possibly think of
A great Mishna Berura source
Lots of Machshava
The Complete Jastrow
Great Hebrew-English Dictionary
Specifically Ben-Ish Chai
Rambam's List of 613 Mitzvot
Over 1000 Links of Torah
A Biographical Study of Rabbi M.M. of Kotzk
Enjoy! Thank you, Yair!