The PostgreSQL Extension Network, PGXN for short, is a website (http://pgxn.org) launched in late 2010 with the purpose of providing a central distribution system for open source PostgreSQL extension libraries. Anybody can register and upload their own module, packaged as an extension archive. The website allows browsing available extensions and their versions, either via a search interface or from a directory of packages and usernames.
The simple way is to use a command-line utility called pgxnclient. It can be easily installed in most systems; see the PGXN website for how to do so. Its purpose is to interact with PGXN and take care of administrative tasks, such as browsing available extensions, downloading the package, compiling the source code, installing files in the proper places, and removing installed package files. Alternatively, you can download the extension files from the website and place them in the right place by following the installation instructions.
PGXN is different from official repositories because it serves another purpose. Official repositories usually contain only seasoned extensions, because they accept new software only after a certain amount of evaluation and testing. On the other hand, anybody can ask for a PGXN account and upload their own extensions, so there is no filter except requiring that the extension has an open source license and a few files that any extension must have.