Exporting a list of your Installed Packages

If you want to create a system that is similar to a different system you have already set up, it can be difficult to remember each and every package you had installed.This method works best when you are exporting to and importing from the same distribution and, specifically, the same releasefor example, exporting from Ubuntu Dapper to Ubuntu Dapper or ubuntu edgy to ubuntu edgy.
Read the blog entry from UbuntuGeek