pNewss is a NNTP news server written in PHP: the PHP News Server. It's simple (less than thousand lines of code), lightweight (no daemon or big caches needed), not too slow, and backed by your favourite database engine (MySQL, SQLite, etc).
pNewss currently support all required commands as a server (including POST and IHAVE, to support feeding from other servers). It supports all required commands as a client (to feed from/to other servers), although some improvements are possible to speed things up.
The sourcecode is available from http://oss.ucis.nl/hg/pnewss/.