UMPCs, or ultra-mobile PCs, are the result of a joint development exercise by Microsoft, Intel and Samsung. They are best referred to as a type of small form-factor PC that fits in the palm -- but don't let their size fool you, they run proper versions of the Windows or Linux operating systems and most software.
To officially qualify as a UMPC, a device must have a screen size of 7 inches or smaller, a minimum resolution of 800x480 pixels, weigh less than 900g, have a landscape display orientation and have a touch-sensitive screen. A full list of specs can be found here.
We've looked at several UMPC-style devices over the last year or so, and not all are created equal. The Samsung Q1, for example, has one of the least impressive hardware specifications of any UMPC. It has a lot going for it, particularly its price, but the lack of an integrated keyboard and very little in the way of extras means it's best thought of as a cheap, shrunken Tablet PC.
The Asus R2H is very similar to the Q1, but it brings several cool things to the party. You get integrated GPS for satellite-navigation and a fingerprint reader for secure logins. It's ideal if you're prone to getting lost, or if you're an MI5 operative with a knack for losing laptops with critical data.
If you asked Inspector Gadget to design a UMPC, he'd probably come up with the Sony Vaio UX1XN. It differs from the Samsung and Asus offerings in that it has a slide-out Qwerty keyboard, which makes it easier to enter text. Or at least it would have, had the keys been a tad larger. The top-spec UX1XN also has a fingerprint reader, a solid-state hard drive (which makes it resilient to sudden shocks) and twin cameras for taking casual photos or self-portraits. We don't believe all these extra features justify the price, however -- at £1,600 it's more than twice as expensive as the Samsung Q1.
If a keyboard is a definite pre-requisite, then you should also consider the OQO Model 2. It, too, has a slide-out pad, which incidentally is far more comfortable to use than the UX1XN's. It also comes with a slightly larger (5-inch) screen, and at 453g it's the lightest UMPC we've come across. The only real drawback is a comparative lack of performance and a relatively high price given its specification.