I was a pretty solid Guild Wars player for years but I haven't played it a couple of years. It's got some great concepts at it and the PvE is a ton of fun but the PvP gets really boring (mainly because ArenaNet sucks at skill balancing and it basically ends up in a "meta or gtfo" situation). I'm not quite sure how active it is nowadays but it's still fun.
I'll advocate for Runescape though, I was a hardcore player in my elementary/middle school years. In retrospect, it's still a good MMORPG. It had a lot of content, cheap subscription fee (if you wanted to subscribe), and being browser-based, it could run on most any system. It may seem like child's play but its solid.
Nowadays I stick more to LAN games like Dungeon Siege (the original) and Terraria (I just play Terraria on LAN). Just get a couple of buddies and find a good dungeon crawler type of game (Baldur's Gate, Dungeon Siege, Diablo II, etc), get Hamachi, and LAN some shit up.