There are plans all over the net to build simple passive resistor-based mixers, much the same principle as building the Y cables previously mentioned in the thread. They give you more room to work and can use pots in place of fixed resistors. Maybe a company or two sells such mixers as well; I know DOD had one.
Myself, I'd get on eBay and get a active mixer with a small number of inputs because it can be useful to have control over levels for disparate computer types, and EQ to get the best out of whatever speakers one has. These mixers can do a lot more besides and typically perform better.
Some like Behringers are cheaper than hell (considering what they are) even when not purchased used.
