|Kindles are awesome (as are other ereaders)|
Check Freebooksifter out:
It's a nicely laid out and well catalogued directory of free ebooks available from the Amazon Kindle Store.
Of course, if you're not using a Kindle or a Kindle app, then you'll need to convert the files and strip any DRM encryption (Hint: use Calibre). It's not as difficult to do as it sounds...