ForeUI provides a set of predefined elements for creating mockup, you can drag them into the editing area and tweak them. You can also create your own elements and libraries.
ForeUI makes prototype skinnable, by changing the UI theme, all elements in the prototype will change their look. Currently ForeUI provides four UI themes for prototyping, including Hand Drawing, Wireframe, Windows XP and Mac OS X themes. These UI themes cover the high, middle and low fidelity degrees, you can switch the UI themes at anytime, thus the fidelity of wireframe will be switched as well. The figures below is the same mockup rendered with different UI themes.
Looks like Balsamiq has some competition here.