Tomorrow is the start of "Read an e-book week" (an event that has been taking place since 2004). Some places to go for special offers:
Smashwords Read an E-book Week Catalogue - nearly 40,000 multi-format books regularly priced at free, and thousands more that are free or deep-discounted for one week only.
Apple iBooks - for iPhone, iPad and iTunes users, a click on this link brings up a selection of free e-books at the Apple iBooks store, organized by categories such as Featured Titles, Fiction & Literature, Non-Fiction, Children & Teens, Romance, Mysteries & Thrillers, Sci-fi & Fantasy, Biographies & Memoirs, Arts & Entertainment and more.
Barnes & Noble - a neat and well-organized collection of thousands of free e-books organized primarily around Fiction, Romance, Sci-Fi and Fantasy, and Mystery. Click the "See all" link to drill down into multiple other categories of free books.
Kindle - the usual site, and note the "Top 100 free" link on the right.
"Buy My Work" at the top of this blog leads you to places that sell (or give away!) my e-books, and there are some which aren't yet included on that page, e.g. these freebies.