Google made an announcement of a limited test of App Script at the same time that Google Wave was announced. Wave gained much of the attention but App Script has the potential to be far more important. Initially App Script is just available in Google Docs spreadsheets for a limited number of users, but this will expand over time.
So what is it?
This will allow mash-ups on the server side, whereas currently mash-ups have resided within the browser. As more web services allow server-side scripting the web really will turn into a fully scriptable environment. Previously applications that would have required complete custom development will just require integrating together of sevices with scripting to provided the desired application.
The potential for this cannot be overestimated, Google Wave is fantastic but with App Scripting it will be just a messaging service that is tied into cloud hosted applications using many other services.