If you're a hardware multiboxer, I'd suggest FlyFF. Gameguard prevents software multiboxing from working, and it's definitely a game that focuses on monotonous level grinding, but I think controlling a party might be interesting, especially if you do an AoE + Heal type group. (Knight + Ringmaster = huge pulls)

God this topic got me thinking about FFXI. I really wish they'd make the game free to play on some kind of advertising system. I bought 5 copies of WoW, I could never justify doing so for FFXI. Pity, it would be a beautiful game to multibox, the macro system is about as versatile as WoW's, plus you could multibox melee classes! /sigh To top it all off, unlike WoW where you can play solo for much of the content, close to none of the content in FFXI is solo-able, so multiboxing would be like the ultimate "Iwin" tool. Instead of spending 2 hours looking for a group and 1 hour levelling, you could start levelling as soon as you log on. That's something very few FFXI players have the luxury of.