For all the interesting hardware crammed into any given eBook reader, they can't really do that much. That is, unless they've got a "Sony" badge on their foreheads, and you've got a little bit of spare time.
Sony's PRS readers run a stripped down variant of Linux, which apparently makes loading homebrew code fairly straightforward. Some users have found ways to change fonts, icons and graphics, as well as to remap keys, and even add rudimentary applications, like dictionaries and and basic games. (Hey, Sudoku!) The Register has a simple guide to the process—it doesn't involve much more than cooking up a special SD card—which, at least at the moment, won't net you a huge amount of new functionality. But the promise is there, and the floodgates seem to have opened, so why not?
There's no way to know if these kinds of hacks will work on the latest batch of PRS readers, because while they're also Linux-based, but Sony hasn't been terribly accommodating to homebrewers in the past. Let's just say it's conceivable that they've done a little code-caulking here and there. We'll have to see, but PRS-505 users, go forth! [The Register via BBG]