"In audio signal processing, convolution reverb is a process for digitally simulating the reverberation of a physical or virtual space. It is based on the mathematical convolution operation, and uses a pre-recorded audio sample of the impulse response of the space being modelled. To apply the reverberation effect, the impulse-response recording is first stored in a digital signal-processing system. This is then convolved with the incoming audio signal to be processed."
Well, I guess I made a mistake. It's not freeware but ... charityware. If you like this plugin, feel free to donate to the Cancer Research UK.
The example below presents, first the dry example and then wet with Cathedral preset
Download: LiquidSonics Reverberate LE