If you are on a 64bit system, you might sometimes stumple upon 32bit programs which need several libraries of which are no lib32-packages. A good example is the Amazon MP3 Downloader.
You can now either search the missing libraries at packages.ubuntu.com or use a tool which does this for you: getlibs. You run it with the desired 32bit program as a parameter and it will locate, download and install all missing libraries the program needs to run.
Download it at frozenfox.freehostia.com.