Yes, I’m aware of AudD.
There is one big conceptional difference between AcoustID and Shazam or AudD: AcoustID can only identify from the whole file (or more exactly the first 120 seconds audio + the duration), whereas Shazam or AudD can identify from sound snippets anywhere in the audio. This is why these services are suited for e.g. identifying audio that is currently played back. lukz once mentioned that limiting AcoustID’s scope to the use case of identifying whole files and hence only needing to index the fingerprints of the start of the audio actually made it possible for him to host this service in a sustainable way (because it drastically reduces the needed storage and processing requirements).
The secondary difference that comes from these different usage patterns is how the detection is tuned. Because AcoustID was created for the use with MusicBrainz Picard and the definitions of an AcoustID rather closely aligns with the definition of a recording in MB the different recordings (like the original, remix and live recording in my example above) will usually end up with different AcoustIDs, while still allowing for some differences caused by encoding quality. Shazam and AudD are more tuned towards detecting the general song, because the detection of a currently played back audio is one of the key goals.