A post over at Slashdot and an article over at Space.com talk about the computational innards of the Spirit and Opportunity rovers. They're surprisingly low-tech, but work well and NASA seems to find them reliable.
From the Space.com article,
RAD6000 microprocessors are radiation-hardened versions of the PowerPC chips that powered Macintosh computers in the early 1990s, with 128 megabytes of random access memory (RAM) and capable of carrying out about 20 million instructions per second.
Not as powerful as one might think, but then again I suppose you don't need much computing power to operate a robot. They did jam it full of memory though.
What I think is more cool is
In addition to VxWorks' reliability, the system allows users to add software patches -- such as a glitch fix or upgrade -- without interruption while a mission is in flight. "Weâ€™ve always had that [feature] so you don't have to shut down, reload and restart after every patch," Blackman said, adding that some commercial desktop systems require users to reboot their computers after a patch
Now if only Microsoft could make Windows do that...