I'll mention the ones that were actually notable on a game design level...
Tomb Raider: Underworld demo
Drove me bat-shit insane jumping from ledge to pseudo-ledge and falling to my death. You can control the camera, but can't spin it all the way around to, y'know, actually LOOK where you're about to jump. Ledges and handles that look like you should be able to grab onto them tend to be blatant lies, leading to a ragdoll death for Lara. I spent a lot of time cursing at this demo, and promptly deleted it upon completion. If they were to fix the camera and loosen up on what is and isn't an "official" ledge type object, it'd be a lot more enjoyable. This is an example of a game I wouldn't have bought without a demo, and I won't buy with the demo either.
Mirror's Edge demo
This was a short-but-sweet demo. It showed off the premise of the game, had some cool experiences, let you finish your mission, and ended. I'm almost willing to give it a try based purely on the fact that the demo didn't make me foam at the mouth by ending in the middle of a boss or something equally dicktastic. I have too much on my plate right now, but this one's on my used games list, I think.
Fallout 3
This game is pure, burning, radioactive love. As a huge fanboy of the original games, I was really skeptical of it being turned into a 3D world, but it seems they got it right. Everything feels like an achievement, from blowing up a city down to picking a lock or shoving a live grenade into someone's shorts (Sorry about that, Mr. Simms). You get little hits of XP for almost everything, and that just makes it constantly fun. The wide-open world is quite awesome as well, and travel/exploration is by far better than the original games, where you'd just click on a map to travel and maybe get a random encounter. In FO3 you actually trek through the wasteland of D.C. and maybe see something interesting off on the horizon, and bam, you're off on a potentially disasterous adventure. I'd be playing this game for unhealthy amounts of time if not for...
WoW: Wrath of the Lich King
Oh gods, help me. It's pulled me back in again. After I'd finally managed to quit a few months ago, the expansion has pulled me back in. I get home, make some decidedly unhealthy food, and play until 2 or 3 AM. This is not a good thing. Hopefully once the initial rush to get to 80 with my rogue has finished, I can get back to a more healthy schedule and actually, y'know, get some work done on my various projects. Anyway, Northrend seems a lot more polished and interesting than Outland was, and the quest hubs are very streamlined. Maybe it's just because I'm actually using an addon called Questhelper this time around, but I'm enjoying my leveling experience much more than I did in the last expansion. Of course, last time I just skipped a zone ahead and started grinding...
Portal: Still Alive
What can I say, it's Portal with extra levels. The levels are devoid of GLaDOS, but overall the experience is as I remember it. Still an awesome game. I'm finding the developer commentary enlightening as well.
Megaman 9
Does anyone remember Megaman being so mind-bogglingly difficult? This is the kind of game that would've made me pitch a spaz as a kid. Just goes to show, nostalgia is more important than the frustration factor.
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Navgraph update: Calling it Ultima.Navigation.dll. Should be able to do something like NavProvider.GetPath(pathinfo, callback). Callback's for the sake of time slicing, etc. But, like I said, WoW has sapped all of my time, so nothing new is likely to come of this for a couple weeks.