lrf, lrx

whatever the prs500 reads natively. Being in the UK but owning a prs 500 i don't use the waterstones or WHSmith sites, nor Borders as they've recently started selling ebooks in the Uk too) as they sell ePub and my prs500 doesn't know anything about that new fangled stuff.
So, it's mobipocket or lit files for me (so I can convert them with calibre) or else the PRS500 native file.