A moderated liveblog or chat room application that can be hosted on any LAMP environment. Makes heavy use of AJAX and JSON, with PHP endpoints used to interact with the database.
A plugin of my own design that seamlessly selects and implements the best available persistent event handler depending on the version of jQuery in use. Complatible back to jQuery 1.0.
Host a proxy service on your own server to allow JavaScript access to data otherwise unavailable due to the same origin policy. Protect this service using AES encryption.