OK, what's on the top of your list. I'm talking about anything that runs reasonably well, by hook or crook in Linux.

Let's see, for me, there's:

MAME (xmame to be exact, with kxmame as the front end.) It will faithfully reproduce any arcade game who's ROM images you can steal...
Nethack's pretty cool
Nexiuz is a nifty FPS
There's the Quakes and Dooms, which have mostly been ported to Linux.
I just got Diablo II running in WINE.