Eador: Genesis is up there on my list, I've been playing it quite a bit since I picked it up. It's not an easy "pay to win" (as in "the only requirement to complete the game is buying it") game like so many AAA titles are these days. But if you're someone who enjoys a challenge and turn-based strategy then this game is without question something you should pick up.
<span class="bold">Uplink</span> it's a simple game good for killing time which has a reasonably low entry threshold but a surprising (at least to me) amount of depth and engagement value.
This is one of those games where "gameplay" and "UI" are not synonymous. It doesn't have lots of flashy graphics etc. but the game play itself is enjoyable. I likely wouldn't spend many hours in a row playing Uplink, but I do find myself returning to it many times for smaller sessions. I've already easily gotten my moneys worth.
Defenders Quest &
Anomaly Warzone Earth I was initially skeptical of both games, a promise of 'innovation' within the arena of Tower Defense honestly seemed like a stretch to me and while each presents a rather unique concept I've played so many tower defense games that I was doubtful whether or not they'd be able to deliver. Turns out
both games do, and I've been quite enjoying being proven wrong regarding these games.
Defenders Quest provides a tower defense that appeals to my nostalgia for games like Final Fantasy, or Dragon Warrior while also giving some amusing story segways to keep things moving.
Anomaly Warzone Earth turns the tables on tower defense by making you play the 'invading hoard' while maintaining the "preserve and enhance" experience of a standard tower defense. Providing a fast paced experience which even months later hasn't gotten stale.
Either game would be a good buy for fans of tower defense games. Alan Wake Providing a strong presence of setting and ambiance the likes of which made Diablo (first game not the series) and Max Payne so satisfying for me.
In fact fans of Max Payne will find a lot to love from the Alan Wake titles (unsurprisingly when you consider who developed both titles). I won't go into too many details because I'd like to avoid spoilers but for those who enjoyed either Max Payne or Momento (movie) you'll likely find something to satisfy you with Alan Wake.
Legend of Grimrock I have yet to play this game extensively myself (the backlog of games GOG has created for me is rather epic) however my brother, the same guy who turned me on to The Witcher, swears by it. In fact part of why I haven't had much chance to play the game is that he's monopolized it so thoroughly even to the extent of playing it over the copy of D3 he picked up.
Any of the above would make fine additions to a gaming collection. :)