From Facebook Developer Wiki

In this article you can find documentation on public (shared) custom tags created by other developers. If you've created new public tags, let the community know by linking to the tags and their documentation here.

• FlashMusic offers custom tags that let you show the flash MP3 player on your facebook program, it provide customized functionality in order to make your own favourite style of flash MP3 player.
• Imgshow offers custom tags that let you specify the keyword, then using Google Image Search API to search the images and show the images in gallery mode.
• Causes offers custom tags that let you show your support for your favorite causes by displaying badges that invite users to join and support the cause.
• Graffiti offers tags that let you draw and display graffiti.
• iLike offers tags that let you display favorite songs and playlists, complete with controls to listen, dedicate, watch, and make ringtones of those songs.
• Visual Bookshelf offers tags that show the most popular books by category.
• Astrology offers custom tags that allow you to provide Daily Horoscopes to your users.
• AppRaise offers custom tags for an easy application link exchange integration.
• Cities I've Visited by TripAdvisor has a tag to display a user's travel map.
• Family Crest offers a tag to display the family crest for a surname.
• Public Tags offers a Countdown Timer.