At the moment I'm running a DC Adventures (Mutants and Masterminds 3rd Edition) game. I'm also running a Rogue Trader (Warhammer 40K RPG) game as well.
I think a lot of GMs have their buffer of campaigns that they want to put into the hopper, to run if they have time. My current buffer looks something like this:
1. Way of the Wicked AP (Pathfinder). I don't miss running Pathfinder that much, but at the same time, I'm really liking this AP thus far, and I really want to give some of my gamer friends a chance to take some newer rules out for a spin.
2. Red Eye of Azathoth (Call of Cthulhu). I've never run CoC, but this adventure sounds so very cool. I might be a bit intimidated to try and pull off something this cool for my first attempt. Plus, I'm not sure if I could pull off the proper atmosphere in a public venue.
3. Something Marvel (Marvel Heroic Roleplaying). Yup, I know. Just a few weeks ago I was saying that I didn't quite get this system. I did a lot of research, read a lot of reviews, listened to some actual play podcasts . . . but I felt like I wasn't giving it a fair shake unless I read the rules first hand.
Reading them first hand, I still have some misgivings. I'll expound at some other date. However, those misgivings are intriguing enough that I want to see if I'm either wrong or can work around them. Strange.
4. The Savage Worlds Variety Show (Savage Worlds). I'd have to make sure everyone is good with this up front, but I had such a blast running random one shots based on various movies and genres last year, I think it would be fun to do two or three, maybe four session adventures based on a movie or a genre, then rotate to a whole different genre or theme for another few nights.
I'm sure this list will mutate and change over time, but this is my current semi-solid list of campaigns that I'd run, if I had the change, at the moment.