ExtremeTech has taken a look at Matrox TripleHead2Go Digital, this product takes one DVI output and drives two or three displays with it.
Dual-monitor setups have been proven to increase productivity (as much as productivity can be proven, anyway). To that end, most graphics cards—even the cheap ones—have a pair of monitor outputs and all sorts of good software features for using two monitors at once. What do you do if two isn't enough, though? The kings of "lots of monitor outputs," Matrox, released a product that would use one monitor output to drive three displays. This TripleHead2Go was reviewed back in 2006, and though it works as promised, it has one glaring flaw: it's analog VGA only.
Fortunately, Matrox has just recently released digital versions of their DualHead2Go and TripleHead2Go products.
You can read the full review over here. The reviewer says the product works as advertised but says it's way too expensive, for most users it would be cheaper to get an additional graphics card.