You'll also find sections describing libpqxx, older parts, or mirrors of parts of libpqxx on these sites:

•  pgFoundry, the development site for most  PostgreSQL add-on projects.
•  Launchpad, Canonical's collaborative development application.
•  Freshmeat, the online software meta-index.
•  Ohloh, a site that tracks software-engineering metrics for free (or open-source) software projects.
•  kirit.com has the Windows porting work by Kirit Sælensminde.

There may be more, but these are ones we are aware of and have some involvement with.