This question on Stackoverflow spurred me to write my first Google App Engine project and consequently my first Python script, jsonip. It lets you retrieve a client's IP address without needing your own server-side script. Normally you could make a call to a local server script that prints the user's IP address then use an XMLHttpRequest to retrieve it. jsonip uses JSONP, so you could use it in a situation where you don't have server-side processing available. The usage is simple enough:
Here's the source code for the Python script:
(When I say this was my first Python script, I mean it! I had to ask Google how to write if..then statements... )