Jeremy mentioned this game in a post a while ago and I didn’t take much notice, but after hearing Justin McElroy on the Joystiq podcast carry on about Kongregate at length I finally caved and took a look, there I stumbed upon Desktop Tower Defence and bunch of other pretty amazing browser based games.
From the DTF Wiki:
Like other games in its genre, the game’s basic premise is simple. The game is played on a set map, in this case one resembling an office desktop. The player must stop a set number of enemies, known in the genre as creeps, from reaching a set point on the playing field. This is accomplished by building and upgrading towers that shoot at, damage or somehow hinder the enemy creeps before they are able to reach their objective. Unlike many tower defense games the path of the creeps themselves is not set; instead, they take the shortest path they can find to the exit, with the towers you can build blocking their way. (The game will prevent you from making an exit inaccessible.)
Very simple, very fun, very addictive.