There are hacks which add the option to change fonts, or read new formats, and hacks which let you play apps and games.
But the one hack I am most interest in is called LazyKindle. Published last week on MobileRead, LazyKindle builds on the late-generation Kindles support for USB Host and uses it to add support for a wireless presentation remote.
This is a very basic type of remote which can be used by someone giving a presentation, but plug it into a hacked Kindle and you can use it to change the page or alter the brightness of the frontlight:
Right now the hack only fully works with the PW3. The page turn buttons work for the Oasis and Voyage, but not the brightness. but that should change in the long run.
The total cost is under $20.