Matrox announced the DualHead2Go and TripleHead2Go which are compatible with a number of Apple systems. Both of them let you drive two or three monitors at combined resolutions up to 2560×1024 or 3840×1024 from a single video card.
The DualHead2Go is ompatible with all recent Mac systems but the TripleHead2Go can only operate with MacBook Pro, Mac Pro, or 20/24-inch iMac. And you may need a mini-DVI to VGA adapter for some Mac system. The prices are set at $169 for the DualHead2Go and $299 for the TripleHead2Go.