Every instance in Amazon's EC2 cloud has both a public and a private DNS name. When dealing with my instances, I often find that I have one of these two names, but really want the other. For example, one of tools lists the status of our servers by their private DNS names (domU-11-22-33-44-55-66.compute-1.internal). If I need to log on to one of these machines to do something, I need to convert this to a public DNS name (ec2-111-222-333-444.compute-1.amazonaws.com). My solution so far to mapping between public and private DNS names has been to simply write down this information on a glass panel by my desk whenever I start an instance.
Needless to say, this solution works fine whenever I'm dealing with my developer instances from my desk, but it doesn't work so well when I'm working from home or dealing with instances other people have started. We have a few general-purpose tools that will pull down instance meta-data, but it's a bit of a pain to use. So, I finally gave in and wrote a simple Ruby script that pulls down all instance metadata and does a simple regex pattern match against the public and private DNS names. Just pass it an initial substring of the private DNS name (eg, "domU-11-22-33-44-55-66", "domU-11-22-33-44-55-66.compute-1.internal", or simply "domU"), and it will spit out the public DNS name for all matching instances. It works in reverse, too, if you want the private DNS name for a given public DNS name.
Note that you'll need to have the amazon-ec2 gem installed, which you can get by simply running the command "gem install amazon-ec2".