Ewa (East-West Audio) is a GPL server program that dynamically adds intros and outros to mp3s on the basis of user-defined rules. With ewa, internet audio publishers can periodically rotate the promotional content in their mp3 downloads without remastering.
Ewa is designed to meet real-life performance requirements; it is extremely fast and scaleable, and the spliced mp3s play on the players users actually use.
Ewa is written in Python and should run on any reasonable UNIX system.