Plogue Bidule and Audiomulch both spring to mind for paid alternatives, bidule being the cheaper of the two.  Although the objects are lower level, one can achieve similar results with Puredata and some of the modular synthesis abstractions that are floating around on the net. PD would take more work, but it has the advantage of being free in both senses, and its cross platform.

There are other options, but these are the three that spring to mind

-michael n