Behringer Powerplay PRO-8 HA-8000 Headphone Amp
This is a good inexpensive amp to drive 8 sets of headphones. 1/4" inputs and outputs which allow you to cascade these for situations where more headphones are needed. For around $150, this is a good product.
Bear in mind that this system provides a single stereo mix to all headphone outputs but separate 1/4" mono phone inputs are available for each amp. These can be connected to AUX outputs on the mixer to get seperate mixes for specific headphones (i.e. the drummer may want more of the click).
Another cool feature is headphone outputs on both the front and rear panels. Normally, you'd only use one but both can be used at the same time as long as they are not too low Z (impedance/ohms). Both phones will have the same level, but it can be handy in a pinch if you need more headphone outputs.