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They seem to have pretty obvious purposes:

http://www.google.com/linux?hl=en&q=boot&btnG=Search

http://www.google.com/microsoft?hl=en&q=boot&btnG=Se...

You can create custom google searches for anything, it's as simple as appending a couple of keywords and terms to the search string or limiting the search to a few URLs. I've used it a few times in the past. Stands to reason there would be a snappier URL for the more obvious and popular searches.

http://www.google.com/cse/

The Microsoft and Linux search "special" searches definitely predate the CSE product. Not sure how long they've been around for, but they're really, really old.

Full list: http://www.google.com/options/specialsearches.html

Also check out the related http://www.google.com/linux (didn't find one for Apple).

Try: http://www.google.com/mac/

I'm an idiot, I saw all the extra stuff without seeing the "Search Mac Sites" text in its search button; in my defense, the other ones don't say anything special in the button.

Also: http://www.google.com/bsd

By far the best one: http://www.google.com/ie

This one is probably specialized for some PDAs. Also this: http://www.google.com/xhtml

Weird Google Search Page