Shapely and Fiona are essential Python tools for geospatial programming written by Sean Gillies.
Shapely does manipulating and analyzing data.
With Shapely, you can do things like buffers, unions, intersections, centroids, convex hulls, and lots more.
Fiona does reading and writing data formats. For this it uses OGR, the most popular open-source conversion system. OGR is extremely powerful and supports many, many formats - it’s used by Mapnik, a tile rendering engine, to support more types of data, and used by people like me every day to convert formats.