Can I have multiple monitors on my MacBook Pro (Early 2011)?
If you want a cheaper alternative to running two Thunderbolt Displays, you can add a second monitor (in addition to one off the mini-Displayport) try a USB Displaylink adapter, like this one from OWC (there are other brands, any one using the Displaylink technology should work).

The performance won't be as good, but as long as you're not doing anything involving lots of 3D, it should be acceptable.