Impulse responses

It is quite the opposite of freeware, unfortunately, but Altiverb 7 XL prides itself on just this feat!

except:
For Bricasti IRs i use these

www.samplicity.com/bricasti-m7-impulse-responses/

In simple terms the convolution program is the software that effects the audio with the impulse response.

The impulse response is the measured response from an impulse passed through a piece of gear or into an acoustic space/instrument.

Other than that it's over my head.

Also in general terms to me an ir reverb sounds a lot better than an algo.

The impulse response file is a kind of extract of a signal processing, that can be mapped on a new signal that is mathematically prepared from a defined input signal and a processed signal sample.
Most of the time, the defined source sample is a dirac function impulse. It is sent to a room or signal processing and the resulting signal is sampled. Then you let a deconvolution program calculate a kind of transfer function file that can be used to map to a new signal.
Then it can be fed to a convolution reverb which then "maps" this characteristic to any new signal.
Convolution is used in many fields of signal processing and got interesting for music the time computers got enough power for realtime processing. The comvolution and de-convolution processs requires mathematical operations that quite some computation power, involving calculations like integrals and fourier transforms etc.. There are different methods of convolution and deconvolution with different computational effort. I do not want to go deeper in the math, it is not trivial and the interested people could google up convolution and will get plenty of mathematical explanation.

The convolution reverb takes your signal and maps the reverb/echo from a room or machine in form of the impulse response file on it.
This costs considerably more calculation power than most algorithmic reverbs, but you need the room or reverb unit only one time, make your dirac sample and after preparing the file with deconvolution you can use it on any signal with the convo-reverb.
Results can be pretty impressing. Reverb/echo is a good application for convolution.

But still, the number of aspects of the room or unit, that can be taken by a convolution reverb are limited. You actually take the information from one sample at one volume level, several aspects like nonlinearities with different dynamic behaviour can not be replicated with this "simple" kind of trick. There are more advanced methods that take series of impulses so signals with different dynamics can get a different mapping.
The Acustica Audio Nebula engine plugins use such a method and can produce even more realistic replications of rooms and signal processings. At the cost if even more processing power demands.
But for some applications the results are not only impressing, they can be undistinguishable. Up to the point where they are identical.
But also, not all aspects can be replicated in some cases, depending on the kind of processing and it's complexity.

For the replication of room reverb of real rooms, results are most of the time better than algorithmic reverbs. But in many cases you might not want a realistic reverb, but one, that gives a distinct color or smear or characteristic that suits the music better than a real room does. So a convolution reverb does not render algorithmic reverbs obsolete. Using the different kind of reverbs on the right occasion can be the key to a great mix.
Say, you give the lead vox a lush old Lexicon algo verb that makes exactly the imperfections (in comparison to a real room) that we want to hear on the voice to make it mighty, and use a real room for the drums in a convolution reverb and maybe you sent all signals except maybe bass and kick to a reverb bus and build up a stereo image and room depth impression that you like...

I bought Reverbarate2 on Black Friday haven't used it a lot yet but first impressions is I think it's quite good , a lot of perimeters to tweak . The Bricasti samples sound better to me than the simplicity Bricasti samples I had in TLSpace. Jmo
Edit: I demoed the Hofa reverb and it was pretty killer ,but they didn't have a Black Friday sale .

here are some more

www.3sigmaaudio.com/free-reverb-impulses/
www.waves.com/downloads/ir-convolution-reverb-library
redwirez.com/free1960g12m25s.jsp?ref=home