We use the Martinsound MultiMax EX system in our studio. It has multiple surround inputs and sources as well as multiple speaker outputs, including small mono. It also does downmixing from surround to stereo and helps you calibrate with the built in pink noise generator.
We have been really happy with this box. We bought ours for about $3K at Sweetwater.
I have a Studio Technologies StudioComm 74/75 system here for 5.1 monitor controller duties. It has two 5.1 inputs, a matrix unwrap function, talkback, dub out and pretty much all other features you'd expect from such a device. The company make simpler and cheaper models as well. I've had good mileage from Studio Technology devices up to now. Since everything goes in XLR or 1/4", better check that soldering iron beforehand. http://www.studio-tech.com/ProductPage-M74M75.htm
Another studio is equipped with a Dynaudio AIR system, which connects all speakers through Ethernet and integrates a table-top controller. That system is super-sleek, perfect for easily jumping from one config to another. I don't like the sound of those speakers, myself, though. Blue Sky also make systems based on the same principle and I remember them sounding pretty good. This kind of integrated system might be worth checking out.