Toolkits are collections of application-specific functions that extend matplotlib.
mpl_toolkits.mplot3d provides some basic 3D plotting (scatter, surf, line, mesh) tools. Not the fastest or feature complete 3D library out there, but ships with matplotlib and thus may be a lighter weight solution for some use cases.
(Source code, png, pdf)
The mpl_toolkits.axes_grid1 toolkit is a collection of helper classes to ease displaying multiple axes in matplotlib.
The mpl_toolkits.axisartist toolkit contains a custom Axes class that is meant to support for curvilinear grids.