How to find .debs (even if you think they don't exist)

One of the biggest strengths of Debian (and derivatives like Ubuntu) is support for the .deb package. After all, it provides a one-click method of easily installing programs. Best of all, these programs are automatically updated via the official Debian repositories. Unfortunately, the official repositories aren’t always the best. Some programs aren’t always up to date (the latest version of Thunderbird is 2.0. However, the latest version in the repositories is 1.5). Worse, some packages aren’t in the repositories at all (Glest is a good example). True, you could build the program from source, but there are a number of reasons why that is undesirable (finding the dependencies, having to download the program again to uninstall it, not automatically adding itself to the menu, etc.). How do you find good Debian software?