Secondary display through network? -> ScreenRecycler
Using ScreenRecycler, you can use an older PC or Mac (say, an iMac) as a secondary screen for your newer Mac. The idea's quite ingenious. ScreenRecycler has two parts. One is a VNC server, the other's a driver (kernel extension). This way, if you start the VNC server, the Displays preference pane shows you a second monitor. From your second machine (whether it's running OS X, Windows or linux), you connect to your first Mac via a good VNC client. Make sure to turn off video mirroring in the Display preference pane and choose the second computer's native display resolution from the first Mac's Display preference pane. Nice. :) It sure doesn't match a real display connection speed-wise, but for many tasks it's fast enough if your network's up to speed. Just give it a try
, the current beta version is free - the final version will probably be shareware (but if you've got a use for it, certainly worth a couple of bucks).
As for VNC-Clients: VNCThing is good on PowerPC Macs, VNC Navigator on Windows and there are several options on linux as well. Some Mac VNC Clients are slow, so you might want to try another one before telling ScreenRecycler off.