There has been a trend lately of apps being released that act more like traditional floating windows. LilyPad HD, OverSkeen, and AirTerm have all garnered a lot of interest, but they are single use apps. Floating Widget does just what its name suggests: it makes widgets float on top of your apps. The implementation is a little odd at first, but once you learn the rules, Floating Widget is useful in a number of situations.
The actual Floating Widget app isn't the most intuitive, and the instructions it offers at start-up are hard to follow until you actually dive in.
If you like to spend your free time hacking around on your Android device, you've no doubt felt the sting of switching back and forth from the terminal. Now with AirTerm you can leave the terminal floating on top of whatever you're doing. This app comes from the same folks that brought you OverSkeen and AirCalc.
This is a true floating window that you can move, resize, and minimize while you work. You can run multiple instances of AirTerm on the screen, and the interface is tabbed for easier management. AirTerm can also install busybox, ssh, vim, rsync, and more on your device if they're not already present.
One of the biggest features of the Galaxy Tab series (minus the OG Tab) is TouchWiz UX and its mini-app tray. In fact, it was difficult for me to get used to the Transformer Prime after having the Galaxy Tab 10.1 because I miss those little apps so much. For the uninitiated, mini-apps in Touchwiz UX are small "portable" applications that run in an independent window on top of the foreground application.
The TouchWiz UX mini-calculator
Of all the mini-apps, the calculator is among the most useful - who really wants to navigate away from the current application to use a full screen calculator (especially on a tablet)?