Oh, the collecting creep has happened to me too...
I started with Judges Guild stuff. Really, I only just wanted a few
JG items. The City States, Dark Tower, Caverns of Thracia... but then I started getting more and more. Now, I'm pretty much done with
JG (just need the First Fantasy Campaign). But while I was collecting
JG, I also started up with Gamma World stuff. Now I have all six editions of
GW (*and* Metamorphosis Alpha!). But I wanted to get a lot of the other box sets that TSR came out with, so I had to get Boot Hill, Star Frontiers, Top Secret... Somewhere along the way, I also got interested in early Chaosium games, so now I've got Runequest and Thieves World, and I have my eye on early Call of Cthulhu and Elric stuff. 'Course, I still collect new stuff too. White Wolf, Green Ronin, Mongoose, Troll Lords, etc.
Actually, eBay helps with this addiction quite a lot. I even started watching and collecting other non-gaming things. I'm now the proud owner of an entire run of the '70s Jack Kirby comic book Kamandi. I'm watching
two-headed wishnik trolls now, for some ungodly reason (actually, it's pure childhood nostalgia). I keep telling myself that I'm only watching them, not buying...