You know, I totally forgot about Thieves' World. Prior to (re-)starting the current Midnight game, I was torn asunder when trying to choose between it and a new, fresh Thieves' World campaign. I've said previously that of all the settings I've helped to develop, that of Sanctuary is the one that I'd most like to play right out of the box.
I am adding several rules from the TW books to my Midnight game, just for grins. To name a couple, I'm allowing players to take levels in the Savant and Assassin core classes from TW, and I'm using the massive damage rules from TW as well. I also intend to implement the rules for Reputation that are presented in TW (so that I can judge how far my players' reputations have gotten, both for good or ill).
Star Wars is another d20 game that I really enjoyed running, back when I was running it. I'm saddened by the fact that it sees little support these days. Some folks might say that the Star Wars collectable miniatures game counts as RPG support, but I'm not really buying it. I don't have the money to invest into SW minis, and even if I did, I'd prefer to fill out the first series of minis (as it contains figs representing characters from the "classic" films). Clone troopers? Meh. Give me a fistful of good old-fashioned stormtroopers any day.
There's also d20 Modern, a game that I've written for (the up-coming Future Player's Companion), but which I have (embarassingly) never actually played. The systems are more or less the same across the entirety of the d20 platform, so I don't feel too bad about it. Yet I would enjoy giving Modern a shot, regardless. I nearly began a Modern mercenary game not too long ago. I don't remember what stopped me; likely my aforementioned attention deficit disorder.