Our gaming gaumet runs from d20 to more eclectic ancient systems, though I am only now about to start playing D&D d20.
Judge Dredd has been big, and exceptionally popular game based around a Civilian Gangster game, sending up the Sopranos and Godfather. Started in January, it ran for 6 full months, with 1-2 sessions of 6-10 hours each week.
The Current Love is 'All Flesh Must Be Eaten' from Eden Studios. Seems one dimensional at first, but is a deceptively well produced game. Zombies have never been so much fun.
Pendragon - This old timer has been running for years now, infact some of the players have been in the campaign longer than the 6 years I have known them. It regualarly takes up about half of a years sessions, and is without doubt one of the best Fantasy RPGs out there.
Legend of the 5 Rings - Samurai Epic, now in its third year, and fourth campaign arc. Samurai classic with a lovely balance between action and political objectives. The system is quite a nice balance between progression and development.
Call of Cthulhu - My personal love and IMHO the best game out there. Certainly the most challenging. D20 is a nice addition to the BPR fold, tho possibly only with a few changes to the core rules.
Others - We have also had Herowars, a herowars pendragon session and several attempts at SLA (a game in dire need of a system overhaul - Fantastic but a little too player dominanted)