Solution 3 is to use magical contract IDs. This can be seen in the case of protocol handlers, which have contract IDs of "@mozilla.org/network/protocol;1?name=" followed by the protocol name. Of course, this isn't necessarily better than using the category manager; it's just another option :)

(From Alex: Yes, I left that one out, and it's a valid point. I use that technique myself in Verbosio for a couple things.)