LAConvolver is an Audio Units plug-in for convolving the audio stream with an impulse response stored in an audio file. LAConvolver is free but donations will certainly encourage further development.
Performance: The CPU-load is dependent on the number of frames per slice, larger slices give lower CPU-load. On an iBook G4 800 MHz with a sample rate of 44,100 Hz and 4,096 frames per slice the CPU-load is about 3% per second of impulse response.
Impulse Responses: Impulse responses are available for free at various sites. You can find a list here.