One of the game that I got as part of my Potato Sack acquisition from Stem was Defense Grid: Awakening. I've got this for my 360 as well, having acquired it last summer when it was on sale for like 200 points. It's an awesome game, but obviously was not one of my biggest factors in acquiring the Potato Sack. I have been meaning to give it a good review for some time, so we'll just count this as a PC and 360 Arcade review since the game is pretty much the same on both platforms.
Defense Grid is a tower defense game - which basically means you have swarms of invaders (aliens in this case) who travel various paths to try and reach a goal (in this game, they are trying to go to your core reactor, to steal energy cores for themselves, which then will then try to march off of the map). Your objective is to create defense towers to stop them. You do this by gaining resources - and these increase a couple of ways. First, you are always gaining, though the more cores you currently have protected (you start with 24 and if all of them are stolen, it's game over) and the more resources you have on hand, the faster that value goes up. So for example if you're sitting on 24 cores and 1000 resources, every second that goes by you may gain 3 more resources. If you have 3 cores and 10 resources, you may gain an additional one every 30 seconds or so. It creates an interesting dynamic where you want to sit on resources as long as possible - but not so long that your defenses are under-developed and unable to thwart the invading aliens.
The other way you gain resources is by destroying aliens. They came at you in a variety of shapes, sizes and powers. Weak herds of them may band together, trying to swarm through fifteen at a time in a cluster, making it hard for guns to single them out and pick them off - so perhaps a flame throwing tower works better here. Boss aliens are better armored and so flame throwing is not nearly as effective as a gun tower. There's faster than average aliens, flying aliens, aliens who spawn more aliens, and it's a challenge to set up a proper defense that can best handle all of these things. A missile tower might keep the skies free and clear, but they don't help you at all against ground units.
Aside from a large variety of towers to choose from, you can also spend to upgrade them. This improves their abilities - but usually at considerable resource cost. It's a wonderful balance of strategy and management, all wrapped up in a visually pleasing sci-fi package. The music's nice enough, and there's a narrator with a calm, likable voice as the somewhat thin storyline is advanced by him and new towers and tools are skillfully integrated into your arsenal level by level.
Time flies when you're having fun, and I've always found this game to be personally very fun. I remember playing it the first time and then realizing around 3am that I had been playing for about 5 hours. I had beaten most of the maps in that time, but there's varying degrees of difficulty, leader boards, and the very nature of the game begs for experimentation and replay as you attempt to protect each and every core the next time. The graphics are not exactly powerhouse, but I thought they looked better than most tower defense games, and this one can be had on Steam for $10.
I already had this one so for me personally the value as part of the Potato Sack was slightly negated, but for fans of this genre I would encourage giving the demo a try. For me the title's a 9 out of 10. Quick video below of one of the earlier levels:
