Other than what I voted (D&D 3.5), I currently use Midgard (PnP) and Talislanta (PbP), as well as (rarely) Mekton Zeta, Stargate SG-1 d20 and Amber (all PnP).
We are using a heavily modified version of Monte's AU, including bits of Skull & Bones, Swashbuckling Adventures, the 3.0 core books, and ideas of our own...
d20 Past doesn't come out until next month, but I'll be GMing that too. And I'd like to be running a d20 Future game or two, but there are only so many hours in a week.