I've been working on a script to upload files over HTTP, and in doing so needed a way to encode data to Base64. The following is AutoIt code ported from a VB5 program. Enjoy.
Update: Thanks to gcriaco's asking, I've cleaned the code up so it should work as a standard UDF library. It's attached here as Base64.au3, and the example to show how it works is here as Base64_Example.au3. Thanks to griaco and trancexx for their help.