Urllib2_file is a project mainly written in Python, it's free.
Extending the urllib2 library so it can handle file uploads and work on Google App Engine
This is an adaptation of the original urllib2_file library at http://fabien.seisen.org/python/urllib2_file/
It can now work on Google App Engine, you can see how I'm using it at: http://swizec.com/blog/programatically-uploading-to-blobstore-in-python/swizec/1423