Since nearly the very beginning of Google proper (aka, not BackRub) they've had a separate search for government sites and for Linux. And also Universities... I vaguely remember both a University of Michigan and a Stanford search. (Larry Page was an undergrad at UMich).

try it: http://google.com/unclesam

it searches the .gov domain, because 'uncle sam' employees can't remember to put site:.gov in front of their searches ?

Think of it as a shortcut.

I'd guess over 90% of Google users (and maybe 99%) don't know about "site:" Hell, most people can't even come up with a reasonable definition for "search engine".

this is even weirder.. http://www.google.com/microsoft

I would guess it's the compliment of:

http://www.google.com/linux

Maybe it's Google's way of saying "we're not even gonna go there"

It's not weird--it's very useful for looking up Windows support info or IE dev documentation, e.g..

Looks like the default XP background, no?