When I started using a Mac, I missed the dual-head video card that was in the Linux box I used to use as my workstation. The DVI connector on the G4 tower was one of the old-school ones that apparently only works with Apple Cinema displays, and the Mini that I use now only has one video output.
So, for a time, I worked with one 1280x1024 display. The Mac experience made up for it, but I sure missed having another head to move stuff to when I'm in DeepHackMode.
Then, along came ScreenRecycler.
In short, it totally rocks! I've set up dual-monitor action at both of my offices.
Here's basically how it works, from what I can tell: There's a userspace application that serves up a framebuffer via VNC. It is joined to a device driver that represents a second display as that framebuffer.
The net effect is that you connect to your Mac with a VNC client on whatever platform you have handy, and use that VNC client as a second display for your Mac. The only difference is that it naturally refreshes slower and more "interestingly" than an actual direct hardware connection to the display. It is very tolerable though; most of the time I don't find the side-effects of using VNC distracting at all.
At the business office, my primary display is a 19" LCD that didn't work for others, but seems to work for me. Other than some ghosting from a slow refresh rate, it works great and is nice on the eyes. Now I also have a 17" CRT, hooked up to an old P3 running krdc under Kubuntu to connect to the ScreenRecycler VNC server.
At the home office, my primary display is a 17" LCD. The only flaw it has is a column of about 20 blue dead pixels spaced out from top to bottom on the left side. Now I also have an ailing-but-still-working 17" LCD, hooked up to an old Dell laptop running XP that previously only was used for dialup connection sharing. It now runs UltraVNC to connect to ScreenRecycler.
It's not the fanciest set of hardware, but having two heads to work with again is comforting -- especially since I have a similar setup in both locations I work from!
If you have a Mac and a spare Linux or Windows machine that is unused as a workstation, give it a try. You'll be pleasantly surprised at what a cool piece of software it is!