Rot-13 Addin is a Rot-13 encoder/decoder for Microsoft Outlook 2000/2002/2003.
If you want to encode/decode text using rot-13, just replace each English letter with the one positioned 13 places forward/backward along the alphabet. The goal of such a simple encryption is to hide part of your message temporarily, and "force" recipient(s) to read the rest of the text first.
You can use Rot13 for movie spoilers, jokes, puzzles etc.