I have played, in roughly chronological order (+ means I GM'd, a number means someone else did) :
(+3) D&D, 4th Edition. Went through the "official" adventures up to Pyramid of Shadows, when everyone got bored. (1st DM'd by someone else, 2nd and 3rd by me.) Also included several of the 3rd E. mini-adventures provided for free on the website, hastily ported (meaning BS'd) into 4th E.
(+) a self-made system based off of 4th E. with a different character building system and more diverse and streamlined combat. (For something I made in a couple of hours, it was surprisingly as good as 4th E.) Never did more than a bit of playtesting, though, as my players decided they hate Pyramid of Shadows.
(1) Star Wars. Worked off of some rulebooks, but mostly improvised. Lasted for one or two dozen sessions, but mostly died off. We could still play, but the GM has moved onto something else.
(+) improvised Fallout: New Vegas game. Pretty fun, but short (two or three sessions), as I lack creativity.
(2) Dark Heresy, without even reading any rulebooks. Went well, but only lasted one session.
(+2) Pathfinder. Played this the most (time-wise; less sessions total than 4th E., but our sessions have been getting longer). I've GM'd 2 or 3 different settings. Another person (2) is doing a heavily Malazan-inspired setting, operating mostly off of the Pathfinder rules for non-magic things; we still play this irregularly.
(1) Unnamed; system inspired by Pathfinder, setting inspired by ASoIaF but pretty original. We've restarted three times due to major setting revision, but we should be starting "for real" soon.
(2) Unnamed; system free-formed, inspired by experience so far; setting inspired by Ghost in the Shell, but almost entirely original. Pretty interesting, works well, hard to explain in short; have played a few sessions, but nothing very recently.
So... Eight games, two of which we purchased rulebooks for. All with a group of three to four people (one doesn't show up very often). It's been a fun few years. Many interesting tales.
I'm currently working on reworking Pyramid of Shadows and maybe Thunderspire Labyrinth to make them actually fun and fit into
another new system I'm going to run. Well, I say working on, but that just means I'll have a few ideas a week, write up some very simple revisions once we've gotten together, and improvise the rest as we go along.
My players have told me I'm the best GM.
(Too bad I'm not creative enough to make many original adventures.)
I'd agree with Nadaka on the point of the books, but I'm not sure I really have room to say much about purchases. I have around $200 worth of material, all but $60 from 4th Edition (the $60 being the Pathfinder book and a PDF of the Bestiary).