I've used Inner Space on both WoW and EverQuest. Not really done much in WoW, but have 3 boxed comfortably in EQ.
I had the main window on my warrior, then 2 smaller windows on the side for druid and enchanter. I had it set up so that pressing the back button on mouse would trigger the fight macro on the two secondary characters (assist warrior, then snare and dots for druid, debuff and slow on enchanter. I then had the forward button to tell my druid to target warrior and cast heal. These commands were never sent to the main window, and I never really had to swap to the secondary characters except for looting the odd item.
It's not free, and you'd need a subscription, but it's like $10 for 3 months, and well worth it if you're serious about multiboxing. Look at