If I buy a windows version of a game on steam, will I own the mac and or linux versions of that same game?
Pretty much yeah. Sometimes there's a separate version (as in it gets its own entry on your games list) but most of the time it just downloads the appropriate version depending on your platform.
An Example of a game with a completely separate mac version: Civilization IV. If you bought that, you'll probably notice it's on your games library list twice, but one won't give you any options to install... that's the Mac Version.
Also, some games have a completely separate version and you MIGHT have to buy it a 2nd time: example: Call of Duty Black Ops