The main benefit to come out of the OLPC was that it shocked laptop manufacturers out of their complacence and opened up an entire market segment overnight. The Asus EEE PC (starting at $300) is the leader, but there is very vibrant competition and new products are being announced every week.
In parallel, desktop vendors have been experimenting with the whole low-cost thing, Walmart's gPC being the best example. Ironically, the gPC not a small form factor machine because apparently the average Walmart customer associates size with performance! Ah, the enlightened times we live in.
At any rate, the best low cost PC in my opinion is the shuttle KPC. And the best way to get it is probably from the third party site zareason.com, with the base configuration starting at $220*. This is a perfectly capable little PC for two hundred bucks—unless you have specific performance needs, if you're paying much more than that for a PC you're getting scammed.
You might think this is just a continuation of the trend in the affordability of personal computing. I'm going to argue that it's something qualitatively different: for the first time ever, your computer is no different from your toaster in that you never change anything on it. Not only do you not mess around with hardware modules (it's been that way for a decade now), you don't even install any software on it. It's just not worth your time. You mostly use web applications and one of two of the non-browser applications that came with your machine. You don't even know what an operating system is. You throw it away after a year or two and get a new one.
*I would recommend upgrading from the Celeron to the Dual Core E2140. Reviews say this chip is almost as good as the Core 2 Duo but much cheaper. And also maybe get 1G of memory instead of 512 megs. In any case, Ubuntu is sleek enough that you probably won't notice.
I'm getting one of these boxes myself. I'll probably do a post about it when I start using it.