PostgreSQL provides several way to integrate with External Projects, both inside and outside of the core database itself. The PostgreSQL Development Group (PGDG) provides some resources toward hosting PostgreSQL projects.
Some add-on projects are considered critical to PostgreSQL adoption. One such category are drivers for popular programming languages that are not shipped with the database itself (JDBC, ODBC, others). Critical projects can request space on git.postgresql.org along with a mailing list.
github is a popular site for hosting projects using the git SCM. It provides some basic project tools such as an issue tracker, wiki, and releases. There is a PostgreSQL mirror at github you can fork to easily create patches against the database source code. github doesn't provide a good way to provide any sort of mailing list for the project. Some projects have combined github for the main project hosting with Google groups to provide an archived mailing list.
Other sites that are possible places to host a PostgreSQL related project at include:
Each of these sites has a different mix of features it supports. Support for the preferred SCM of the developer is often the first thing considered, since some of these sites only support one of them. For example, github only handles git, and Launchpad only supports Bazaar.