Reader Marcel sent us this great idea that a few players (including Siij of Laughing Skull) have had around the realms: PvP Bingo. Basically it's all started on a forum thread, where a square full of players (who have consented, I assume, although it might be funny if they didn't) are laid out as targets on a bingo card. Kill a player, mark the card, post a screenshot, and the first to finish up a line wins. Fun.
Here's another one on a different realm. I especially like how some instructions are clearer than others (one of the instructions says to just kill a member of a certain guild), though it's not really fair to put just one guy in the center. Then again, maybe he wants to be there-- would you enjoy having a big price on your head, and players gunning to empty your health meter? Zerging groups are apparently acceptable, and though tracking via friends or alts is allowed, I'd think that would be a gamebreaker. Apparently switching to alts is acceptable, too-- all the kills don't have to be done with the same character.
This kind of thing almost makes me wish I played on a PvP server, although I'm sure the same thing could be accomplished via duels. Fun idea, and definitely should make for some good world PvP. Makes me wonder, too, if Blizzard would ever consider player-created bounties ingame-- while there's probably a lot of potential for griefing, players who consented to it would probably have a lot of fun.