three come to mind immediately:
My entire Everquest experience. I came into the game with a desire to collect and sell the rarest of items, and focused heavily on that--had a friendly guild who used to tease me for being alone in Blackburrow farming gnolls while they grouped in Everfrost, but when I'd get that uber drop and make a ton of gold they got my point. Eventually I had complete sets of armor only few others had, and was filthy rich (being able to afford anything you want off auction is intoxicating). All done with in-game items and in-game gold.
Phantasy Star Online, on the Dreamcast. It was my first online experience, and the rare trading became addictive there, also. I used to log on, have a few beers ready, turn on '80's Night on WPLJ, and get into it. Imagine my shock after tekking the ?SPECIAL WEAPONs I'd looted and finding a Spread Needle!!! This was the ultimate weapon at the time, ultra rare, and I'd found one, totally unexpected. I was probably beyond lit at the time, and I'm sure I wept.
GTA San Andreas, all-time favorite game. SO much to do, a huge world (I wish I could somehow upload some of the in-game photos I've taken with the camera you can find). Found every possible glitch in the game, and finally even saw the Hot Coffee stuff via a cheat device (long after beating it, of course)> I even had the Sindaccos, a gang origianlly intended to populate the streets of Las Venturas they way the other gangs inhabit Los Santos, pop up in my gamesave (they attack out of nowhere carrying Desert Eagles and are many different NPC models rarely seen outside of the missions they are in). Everything from collecting rare cares to awesome two-player stand offs against the cops in LV atop the pyramid...awesome game.
I've lost some of the interest I had as a kid, but some games still have awesome memories attached to them.