No idea how long they’ve had this service up, can’t find any info about it on Google itself, but http://www.google.com/linux is pretty neat: Only searches sites that are about Linux.
Hmm. There seems to also be a http://www.google.com/microsoft. And a http://www.google.com/mac. No http://www.google.com/oracle, though. But there’s a http://www.google.com/bsd. I wonder where the complete list is?
Aha, it’s here: http://www.google.com/options/specialsearches.html.