Audiodia gives you access to the AudioUnit filters that ship with Mac OS X, such as the Reverb and the Delay filters. It also ships with a built-in pitch-shifting filter. Audiodia lets you pick favorite filters and quickly switch from one filter to another during use. The sound output is mixed with your other applications' sound output, so you can add your voice over music playing in iTunes, for instance.
Audiodia costs US$10 and a demo version is available.