I've been using the latest model from the folks at LapWorks - the Futura - for a while now and was delighted to see that every attendee at the recent Gnomedex conference received one as a premium for attending. It's a nice sturdy unit that provides two form factors - a flat desk surface when you have no desk or table available and an elevated wedge stand when you do. The cutouts provide a lot of air circulation that keeps both my MacBook and Lenovo X61 Tablet PC nice and cool (both tend to get pretty toasty) and there are nicely spaced rubber bits arranged on the work surface to hold the laptop in place even at extreme elevation angles.
At $29.95, the price is right and the unit is compact and light enough to fit in my gadget bag with no problem. This unit is ideal for more compact laptops like the ones I'm using. If you have one of those honking big 17" desktop replacement units, you might want to look at another model - LapWorks offers a number of different products for both mobile and desktop use.