Yeah, like matt b I have a chunky old iriver H340, which I think I already wrote about somewhere upthread... I chose iriver because iaudio didn't exist at the time, and because it had a colour screen, audio in etc. and free upgrades.
I run this alternative operating system on it called
Rockbox. This is available for recent iaudio and iriver players (as well as iPods and so on), so if you're happy to give this a shot don't fret about the features of the software, just the hardware, when choosing a player.
With Rockbox you can play and record uncompressed WAV, can play MP3, OGG, and FLAC among others. You can make on the fly playlists that you can save / edit / merge / reorder / etc. really easily; the thing works as a flash USB drive if you want to drag crap around; you can view pics; you can read text files; and you also get dinky little games (I'm addicted to sudoku at the mo).
You can also skin it if you're into that kind of mallarky. You could even make it look like iPod's OS, just with way more options...
Man, you can even change playback speed if you're feeling oh so zany... I've been listening to all kinds of things at different speeds recently, pretty good times... make your own impromptu screwed versions of tracks.
Anyway, I think Rockbox is pretty ace. It's also a dual boot thing, so if you ever want to switch to the original player's OS you can do so just by holding down a button when you turn it on.