A small group of fans (thanks morten, %gone, quazz, chc, krad, demo and frost!) have taken Tony Hawk's Underground 2 and modded the pants off it, adding a bunch of levels and getting the whole thing running in 1080p.
They've also improved online play, added new skater customisation options and even included settings that'll change the game to play like either the PC or PS2 versions.
That's what they've currently got running; stuff they want to add in the near future include levels from every other Tony Hawk's game (!), a level editor and even a new story mode.
Tony Hawk Mods [site]
Openspy[can run PS2 & PC games online via PC]