A variety of usb adapters (About 10+ over the past 2 years as the lads keep breaking them, god knows how) and wireless built into the laptops (had 2 or 3 laptops in the past 2 years).
The ones they are using now are pretty generic wireless-N £10 adapters. When i lived with them we had like 5 computers and 2 laptops running cheap belkin wireless G's and it wasn't as bad as what my brother experiences now.
For example, my brother uses this one
http://www.ebuyer.com/155640-tenda-w...ter-w322u-v2-0
As for wireless device they are using the superhub and the dlink dir-615 with both wireless's on. (They still have issues if its only 1 with wireless on)