Stargazer (http://stg.dp.ua) is a powerful billing system. PPPD is a famous PPP daemon for Linux systems.
Purestg2 is a bridge between Stargazer and pppd. Purestg2 allows you to authorize PPP connecting users with Stargazer.
More detailed information can be found in README.
Discussion in Russian and Ukrainian: http://local.com.ua/forum/topic/25760-purestg2/
Latest version: 2.4
Ubuntu packages available: https://launchpad.net/~lion-simba/+archive/ubuntu/stargazer