I have a few in my house.
My roommate has an old kit-bashed Athlon 64 X2 running Ubuntu with a GeForce 9800-series card driving dual 20” flatscreens. A little long in the tooth, but it still holds it’s own.
My wife and I share a Gateway Core i3–530 with Win7, 6GB RAM, and a fanless GT240. It’s pretty decent for the price, and if it gets too slow then I’ll just slap an i7 in there. For the price, it’s a pretty solid rig and I would recommend it to someone on a budget. I almost built my own, but it turned out that this tower was better than anything I could make for the same price, especially given the cost of Win7.
I also have a Toshiba T135 laptop with a dual-core SU4100 and 3GB RAM than usually does WIn7 but occasionally Ubuntu. It’s a pretty nice rig for $500, and I love the battery life (never less than 6 hours even under hard use). I would not recommend it solely because it has been superceded by the T235, though if you can find a good deal on the T135 with the SU4100 CPU then go for it. (But avoid the SU2700 and the AMD models like the plague.)
My Droid X almost qualifies as a computer (close enough to be worth mentioning) since it does HD video, has an office suite, file manager, and task manager. I love that little bastard!