I'll second Combat Arms since I'm hooked on that at the moment.
For a very well made offline single player game I gotta mention
Portal once again. It's just a great thinking game that was unlike anything I'd ever played before. It's short, but sweet. Plus it has the greatest ending credits song ever.
Another one that's a favorite of mine is
Giants: Citizen Kabuto. It's relatively old, but it has held up well and is still very fun and funny. It's a hodgepodge of sorts that mixes multiple genres, but it works. You can probably find it for dirt cheap these days, or in torrents if you're less than honest.
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