I've found a way to do convolution for free ( stop me if you've heard this one before
)Download and install Soundhack, open the audio file you want to convolve, hit Apple-C. The Convolve menu appears, asking you to pick an impulse. Navigate to the impulse you want, click on 'Process' and the job's done.
Soundhack is an interesting and useful tool in its' own right, convolution being just one way it can mangle sound files.
One recent experiment was convolving the naturally-delayed sound of a public speaker with that of a freight train starting up- did some damn weird things to the voice, all that clanging and screeching 