I've got a fictional world and an outline of what happens in it which I've been tinkering with since I was a teen. Based loosely off of some RP I did before I ran into the DL. Nothing's ever really come of it, though I've tinkered with P&P RP, jRPG, SRPG, and just writing a damned book at various points in regards to it. Got a ton of notes scattered through a few notebooks and a binder.
That said, these days, a lot of my creative energies are drawn into role playing. Nothing that inspires pet project the same way that the old stuff with Cyril did, but it's still a lot of fun and fairly creative.
Right, biggest thing in my life that would qualify as a pet project. My D&D Campaign. I've also run those things sporadically, and the one I have going right now has been a blast, actually. First time where I've managed to get the story going right and the players are all also having a good time. While I definately make the occasional slip, it's not so hard to recover from them. Also, players are not always predictable, even when you have ones that are good natured and not likely to go and do random things just to watch you dance. (Of course, being players, they will still do random things for a million other reasons)