TotalBiscuit seems to like Sang-froid, which is a different take on tower defense, except without towers. It seemed interesting and pretty tactical. It's got pretty awesome reviews.
I had a blast back in the day playing Stronghold Crusader, which is an improvement on the original Stronghold. It's a nice RTS, with some economy, where you build a castle and attack the other person's castle.
Cubetractor I don't know anything about.
Unity of Command is a wargame that everybody can get into. Amazingly sleek, to the point, and just overall awesome.
Eador: MotBW is the same as Eador: Genesis but with a more compact, nicer campaign, but the gameplay is the same. Turn-based, fairly complex, lots to do.
Spacehulk is apparently good if you're into the boardgame, not so good if you're not. People love it or hate it.
Ironclad: tactics is from the guy who did Spacechem, and is more accessible, has a deck-building mechanic to it can be played coop, and is apparently fairly deep.
The fact that you can get these DRM free means that you can very likely play it with a friend by buying a single copy. I'd definitely check the some gameplay footage of the games out, but it looks like the kind of bundle that can make quite some "free time" disappear