I dearly love my App Engine Console. It is a mini-app, a "plug-in" for other applications. It gives you an AJAX Python interpreter that runs on the server—great for development and debugging. Sometimes I use cURL pointed at App Engine Console to run (authenticated) one-off remote Python procedure calls.
(I wrote it, yes, but I use it daily for maintenance of busy sites. It's the first tool I install on new projects and I don't know how I could live without it.)
