The uWSGI project aims at developing a full stack for building (and hosting) clustered/distributed network applications. Mainly targeted at the web and its standards, it has been successfully used in a lot of different contexts. Thanks to its pluggable architecture it can be extended without limits to support more platforms and languages. Currently, you can write plugins in C, C++ and Objective-C. The "WSGI" part in the name is a tribute to the namesake Python standard, as it has been the first developed plugin for the project.
So essentially it's a production-quality WSGI server, so it's what you run your Python Web Framework apps with, instead of the lite/dev server that's part of the framework itself. And you generally run it behind something like NginX (that's what I use).