Slow hash algorithm that does not require salt?
You can use any common slow hash construction that takes a salt with a constant salt. This includes scrypt, bcrypt and PBKDF2.

You obviously lose the benefit of the salt. Since the purpose of a salt is preventing multi-target attacks, now an attacker can attack all hashes at the same time. I'd use at least a salt that's unique for your application, so an attacker can't attack your application at the same time as others.

Personally I'd go with scrypt, since it strengthens the most for a given derivation time limit, but you can use bcrypt or PBKDF2 if you prefer older/more popular constructions.