Available at SourceForge
You can download the released HUPnP versions from here. In addition, you can checkout the latest sources from the HUPnP's SVN repository:
svn co https://hupnp.svn.sourceforge.net/svnroot/hupnp hupnp
The trunk of the SVN repository always contains the latest code, the bleeding edge if you will. This is where the active development is done and backwards compatibility is not maintained here. You should not rely on trunk code where any type of stability is important.
The HUPnP v1 API is maintained in a separate branch.