Aside from Muppets, I collect Star Wars, but not just figures, everything I can find! Star Wars is still my primary passion, but Muppets is pretty easy to collect as well since it isn't as invasive as Star Wars. I have one shelf full of Muppets, two rooms full (dedicated completely) of Star Wars.
Im also going to collect the new Faces of Springfield figures. They are basically 9 inch versions of the World of Springfield Simpsons figures, the first three were recently released. I had at one time collected the WOS line, but it got out of hand, and I sold them. Now I wish I hadn't left it, so Im getting into these to satisfy my Simpsons craving, instead of spending hundreds, if not in the thousand, of dollars trying to get all of the WOS figures again.
I can't remember a time where I wasn't into action figures, especially Star Wars. I grew up on Star Wars, Empire was the first movie I ever saw (I was two, but I do remember Yoda!), and I've had Star Wars figures my whole life.
I've also collected through the past:
GI Joe
M.A.S.K.
Monster in My Pocket
M.U.S.C.L.E.
Battle Beasts
Masters of the Universe (the new figures look pretty cool)
I have to say, of all the toys I ever collected, the Muppet line has the strongest twist ties