The great free book resource,
ManyBooks.net now has a mobile device friendly version called
mnybks.net. It's perfect for grabbing a new e-book while you are on the go with an internet connected device. In fact, if your device has an SD card reader, you can even pop a Sony Reader friendly version onto an SD card and then plug it right into the Reader.
E-books are not usually huge files, but always be aware of any data charges that might pile up.
Via David Rothman at
TeleRead.org.
Update: As noted in the discussion below, the functionality is currently limited to .jar files right now, with the typical cell phone and WAP browser being the initial target audience.