Are there any adapters or "hacks" that make it possible to use a second display on a Mac mini G4?

The PowerPC-based Mac mini models have a single DVI port for an external display and do not support a USB 2.0 to VGA or DVI video adapter like Intel-based Macs.

However, the PowerPC-based Mac mini models do support the Matrox DualHead2Go, which is quite a bit different than a USB 2.0 to video adapter but achieves a similar dual display effect.

Photo Credit: Matrox, Inc.

In the company's own FAQ, Matrox explains:

If instead of plugging in a monitor to your system's video output you plug in a DualHead2Go, your computer interprets this device as one double-wide monitor, so it outputs a single extra-wide image. As you have two monitors connected to your DualHead2Go, the DualHead2Go takes the left half of this extra-wide image it receives from your computer and sends it to your left monitor, and sends the right half to your right monitor.

In basic terms, the DualHead2Go adapter effectively "tricks" the Mac mini into thinking it still is connected to a single display. On the Mac mini G4 systems it reportedly supports two 1280x1024 displays for a combined 2560x1024 resolution as well as a number of other PowerPC-based and Intel-based Macs at a variety of resolutions. Note that both displays have to run at the same resolution and refresh rate and the company recommends that both displays have the same maximum resolution.

With a retail price of US$169, the DualHead2Go isn't cheap -- and most Mac mini G4 users likely would be better off putting the money toward a new Mac mini with built-in dual display support -- but it is still worth considering, particularly for those with substantial investment in PowerPC native (or only) software.

For additional information on the DualHead2Go, the always excellent AnandTech provides a review that may be of interest.