Or if you have any of the C7x0/C860 series...
pdaXrom. It's more polished than Pocket Workstation. And for the 5xxx series, I'd suggest staying away from X for the most part. It doesn't work that well on a 320x240 screen. If someone's done some work on X on the 6000, it might work out, but the 6000 series is not widely developed for.
So, best ROMs in my opinion...
5000D/5500:
OpenZaurus
5600 (shameless - I make this one, but it is said to be the best):
Watapon
6000: Default, as there is no really well working alternatives, that I know of
C7xx/C860:
Cacko (there isn't really a homepage for it, but there are some pinned topics on it there), or
pdaXrom
C3000: Default, as I don't think there are well working alternatives either
On the Zaurus note, my own ROM (Watapon for the 5600s) should have a new version on Friday.
A nice new version.