Fictionwise currently lists 42 ebooks on their freebie page.
Finding Free Ebooks Blog is devoted to listing "websites LEGALLY distributing free ebooks, mostly novel length fiction."
Free Ebooks Blog also keeps track of the various freebies being offered, along with download requirements and any expiration dates.
Wiki.mobileread.com has an extensive list of free read links here.
Download unlimited free ebooks anytime at FreeEbooks.net.
From Kindle 2 Review, here's an extensive list of free ebooks for Kindle and Kindle 2.
The "most imitated" free ebook site, OnlineFreeEBooks.net, seems to have a lot of nonfic and manuals to offer.