Visual Tooltip is a very nice looking addon to your desktop. When you hover your mouse over an item on the taskbar, a small preview of that window pops up. There are a few features, however, that I'm fairly sure Vista does not have (although I'm not sure about Mac.) The first is that you can "pin" a pop up. If you click a little pushpin, the tooltip will stay where it is, "pinnned", although it won't change along with the window it is previewing.
Another feature is the ability to grab and drag the preview tooltips anywhere across the screen. That means you can have a little preview of the window anywhere on your desktop, always on top of everything else.
The last nice feature is the "dock". You can "dock" your thumbnails in a little space, and customize it by changing how rotated the window previews are, what color the dock, etc. You can even make Visual Tooltip essentially take over your taskbar by having the dock include the tray icons, and hiding the taskbar. Visual Tooltip can essentially be a dock program, in addition to the tooltips.
Otherwise, the program is fairly straightforward. There is tons of customibility, anywhere from the size of the tooltip, to the color of the title in the thumbnail, to what type of animation you want. It's a very versatile program.
The program itself is only around 1mb, so I'd say that's pretty tiny. It's portable, as well, meaning you don't have to install. The one thing I do have to say though is that when you first download it, it's in French, but all you have to do is go to "Options", then the last tab labeled "Langue", and change it to English, and you'll have no trouble.