I recently came across GridVista while looking for a tool that would help me manage my open windows better. I cannot afford multiple monitors and find that I often need to have two windows open at the same time so that I can read one, while I write in another. You can split your monitor into two (half size), three (one half, two quarter) or four (all quarter) sections. You can then "maximise" your windows into any of the sections so that no screen real estate is wasted by simply dragging the window into one of the sectors. Don't worry, you can still maximise your windows to full screen and you can select which windows should not adhere to the space constraints set by GridVista, which windows should run on top of the others and even the transparency of the windows.
I have not tried this, but it does appear that you can use it across multiple monitors, configuring each one separately.
I have been using it in Windows XP without problems and Joshua at gHacks tested GridVista after I informed him about it and reports that it works well in Vista. It would be ideal for people that have large monitors (such as 22" and larger), although I quite like it for my 17" LCD.
If this sounds like a great idea, download it here.
UPDATE: A reader at gHacks called Mike suggested WinSplit Revolution. I am not sure which is better, but it also looks promising and can be installed as a portable application.
Update (2009/10/22): I have been using WinSplit Revolution for some time now. If you have a keyboard with a NumPad then this is one of the programs I cannot imagine living without. Outstanding!