Apocalypse World (and hacks!) being are current favorite. I'd love to run The Regiment, Streets of Marienburg, or a World of Dungeons game!
Burning Wheel is my favorite FtF game, but I'm unsure if it would work in PbP.
I'm also OSR: Mainly B/X, BECMI (or clones thereof), and Small But Vicious Dog: I likes-me some Warhammer setting.
I think I found this site from a Lady Blackbird game that was linked to a while ago and I'm just now posting here.