Maxivista allows you to install virtual display drivers on your computer. It then uses other computers to act as your extended monitors. This means, if you have 3 computers and 3 monitors at your disposal, you can install MaxiVista and have one of your machines think it has 3 monitors (with all the goodness that comes with that). The new version has a couple of new features, one of them really really neat.
But the explanation is very convoluted, so I will try to do it step by step:
Say you have three computers - C1, C2 and C3 and three monitors - M1, M2 and M3. M1 is connected to C1, M2 to C2 and M3 to C3. Say you mostly work on C1 and want to have more realestate in it. You go and install the MaxiVista server on C1 and install MaxiVista clients on C2 and C3. Now, when everything is properly set up, C1 will have three monitors at its disposal - M1 through M3 - and you will be able to drag windows around them etc (the performance is impressive btw - you can watch videos and everything).
This is what the classic MaxiVista used to do. What the new version allows you is to change the slave monitors to either show the extended screen of C1 or show the actual contents of their own computers. The nice thing about switching is that you can now control C2 and C3 w/o using a switchbox - MaxiVista will relay the keyboard/mouse commands to C2/C3 through C1 and it does it in the way you would expect - as your mouse moves to the area between M1 and M2, for example, when it steps over the gap and moves into M2, your keyboard will now control M2 instead of M1.
Kind'a complicated to explain - but they have graphics in their sites that make it all easier to understand.
What I personally really like about it is that I can use both my Dell Ultrasharp LCDs (BTW: Dell Ultrasharp LCDs for the proverbial win!) with DVI w/o resorting to installing an extra PCI card.