Robinson Crusoe is a solid game for solo play and just gets more interesting with more players (up to four).
Urbion and Onirim are nice cheap semi-abstract card games with a small footprint .They are both solo by design but also play quite well with two (I actually prefer Urbion with a partner). Both are set in the same universe and have been reprinted so I believe they are (finally) available to buy now.
Yggrdasil is a gorgeous co-op action selection game where you play out a battle between ancient Norse gods. It plays from one to six players quite comfortably.
Mice and Mystics works fine solo, though I find it mechanically weak/repetitive. The bulk of the fun (of which there is a ton!) therefore comes from the storytelling elements which I personally prefer with a group, though YMMV depending on personal taste.
Mage Knight is a spectacular solo puzzle, but it has extensive setup and is a bit of a brain-burner with all the different components to keep track of. A personal favorite, but you may not like it given your solo Tactics experience. Plays up to four.
Space Hulk Death Angel is the game to get if you hate yourself and never want to be happy playing a game again. Not really - tons of people like it and it makes for a very straightforward solo game, but I personally found it brutally hard. Pretty sweet if you like the Warhammer theme, though. Plays up to six.
There's a whole host of games in the cooperative beat-the-ticking-clock area-control genre that play great solo. Theme differentiates them more than mechanics: Pandemic, Forbidden Desert, Forbidden Island, and Defenders of the Realm are top examples here.
And, finally, there are the competitive games that are basically multiplayer solitare and thus don't really need a second player to compete against your own high scores: Race for the Galaxy and Dungeon Roll, for example.
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