Apple - 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.

Any 2011 MacBook Pro model will support Thunderbolt. You can daisy-chain 2 Thunderbolt displays. You can also connect one mini DisplayPort monitor to the end of a Thunderbolt chain and connect your Mac to your Thunderbolt Display.

A non-Thunderbolt solution if you don't have a Thunderbolt Mac: a DisplayLink-style USB to DVI display adapter.

You can always try the Matrox DualHead2Go Digital ME. I use the Mini DisplayPort version and it works well except it can get quite fussy when I try to resume from sleep. Of course, I'm using it with the MBP lid closed; it doesn't seem to have this problem when the lid is open.

One other note: the device makes both monitors appear to the Mac as being a single, giant monitor. However, I haven't really had any issue with this myself.