There's a bunch of free-to-play games you can try multiboxing, depending on how much time you want to spend playing.
If you like Action RPGs and want some casual action, Marvel Heroes is new, easy to multibox and easy to get into. Diablo 3 is popular to multibox, particularly considering the real-money Auction House and the ease of multiboxing it, has no recurring subscription fees but is not F2P -- you do have to buy the game.
RIFT recently went free-to-play so a
lot of people are trying that out right now and now would be a good time to start trying it.
Lord of the Rings Online recently had a big content update and is probably one of the better F2P MMORPGs.
Star Trek Online recently released its first expansion. STO's space mechanics (it has different mechanics for ground vs piloting) are free-flight in contrast to EVE, you might enjoy trying it out.
Neverwinter just completed the Beta phase and is now live and free to play, and there's a lot of activity here on the ISBoxer forum from Neverwinter players if you want to get started. This game is harder to multibox than the others listed.
There's others of course but these are all F2P with recent releases and are pretty well understood by ISBoxer users
