There is quite a lot of mockup apps around whereas only some of them are under active development and additionally are offered with a non-commercial licene. I (only rawly) evaluated two of them, namely pencil (2.0.3) and prototyper-free (2.1.0) for an application in a web-ui project.
pencil is a classic in the mockup scene and may became most popular by shipping a firefox plugin variant that allows for quick web-based mockup production. however, since firefox meanwhile outruns any developer by updates, the current plugin rendered incompatible with my running firefox installation and i settled for the windows installation package. prototyper-free is, on the other hand, a branch of the commercial, fully fledged mockup tool that has been restricted in functionality and more specific in extensionability (see below) without preventing you from getting your mockup job done efficiently.