In addition to many of the sites listed above, I frequent the following three user-curated freebies lists in the BN.com forums: "
Free NOOKbook summary thread - please no OT" (NOOK Book freebies), "
The Official OT Free NOOKbook summary thread" (off-topic posts as well as postings about freebie content, including audiobooks, available outside of the NOOK Store), and "
Smashwords freebies worth considering" (self-explanatory).
Inkmesh.com is also a good source, when it's up and running.