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A bit disappoint by the narrow point of view. Atlas of Number Theoretical Objects would have been more appropriate. Still nice, though.

The site could use a better name. But presumably there's no reason the interface and cataloguing system can't be extended to other domains.

What are some specific examples of other types of objects you would like to see.

All the finite simple groups would be nice, for example.

I assume you mean the sporadic groups? Because the families are all unbounded.