Common Short Code List
Mike just pinged me about a list of premium SMS services, and I had remembered seeing something like that in the past, but couldn't quite remember. (Not unusual for me). It took me a bit to wander through my memory, but I remembered I was checking out the Wikipedia article on Common Short Codes and at the bottom of the list there was a link to Shortcode.info, which claims to be a "global shortcode directory." I don't know if its complete, but it looks pretty good - has tags and everything.
There's also US Short Codes WhoIS which has a nice table to check out, with URLs, etc. Probably a lot of duplicate info, but always good to have another resource. And if you're interested in actually getting a shortcode for your business here in the U.S., you can check out the Common Short Code Administration's US Short Codes site, which explains how to apply.
Now I don't have to try to remember where I saw this stuff next time. Blog as backup brain.