PC game classics/essentials at $30 or less

[XboxMaster]

Open for some recommendations of PC gaming classics/essentials that are $30 or less. I don't find MMORPG's too appealing, but I like FPS's and strategy/RTS games.

[kakomu]

Half Life and Unreal Tournament spring to mind. I think you can get both for a combined $30 (UT for $10, HL for about $20).

Try and find Lucasarts Archives full of their older SCUMM games. You can also get SCUMMVM and beneath a steel sky for free, which is another SCUMM-like game.
http://www.revolution.co.uk/_display.php?id=16

System shock 2
Duke Nukem 3D (Atomic Edition, so you get the 4th chapter), as well as Duke Nukem 1 and 2 (both great games).
Diablo (or 2)

There are bunches more, and I can't think of any off the top of my head.

[RedvsBlue]

Basically anything Blizzard can be found for a decent price and they almost always have high quality.

[AFStealth]

How about RPGS? Knights of the Old Republic should be played and enjoyed by all.

[SOSTrooper]

The Thief series is worth the price too. You can probably get all 3 games for around $30.