Anyway, the very anti-climactic number one.
1. Runescape (Browser based)
If you don't know what Runescape is, it's the largest MMORPG around and basically a poor man's World of Warcraft. I've wasted an embarrassing amount of my life on this game and keep coming back for more every so often. I can't even explain why I find this game so entertaining, but I do, even with the many adverse changes over the years. Anyway, many a fond memory has came from this game, so I'd be doing my childhood an injustice by not putting this at the top of the list.
Fun fact (not really):My username actually came from one of my original usernames in Runescape which was derived from a monster in the game (Kalphite Guardian), but shortened to accommodate RS's 12-character name limit.