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.