Π-Base – A community database of topological examples
topology.jdabbs.com
topology.jdabbs.com
If you're interested in staying abreast, sign up for the newsletter (http://jdabbs.us9.list-manage2.com/subscribe?u=f81a0c2648f40...). If you're interested in getting involved, let me know (https://twitter.com/jamesdabbs).
(I get it's math-related, but what is it useful for?)
The other main use case now is educational - I definitely would have liked a site like this when I was taking my intro to topology class and grappling with examples to get my head 'round things, and I know of a seminar that ran using the db as a hook to get students being self-directed, exploring, and contributing more than undergrad mathematics usually allows.
Admittedly, that may not seem "useful" unless you're a topologist.