XML-Atom-SimpleFeed (CPAN) is a general-purpose Perl module for quickly generating valid Atom feeds, in the style of XML::RSS for RSS. You can see it in action at Iowa Student Computer Association BBS (ISCABBSEst.1989), providing the most recent 30 posts for all public forums, updated every 5 minutes.
A parser for the multiple XML RSS formats that use the elements such as channel, item, title, description in different ways. Attempts to turn the input into RSS 1.0 RDF triples. True RSS 1.0, as a full RDF vocabulary, is best parsed by the RDF/XML parser. It also generates triples for RSS enclosures.
Raptor is a free software / Open Source C library that provides a set of Resource Description Framework (RDF) parsers generating RDF triples from RDF/XML, N-Triples, Turtle, several XML RSS tag soup formats and Atom 0.3.
Raptor was designed to work closely with the Redland RDF library (RDF Parser Toolkit for Redland) but is entirely separate. It is a portable library that works across many POSIX systems (Unix, GNU/Linux, BSDs, OSX, cygwin, win32). Raptor has no memory leaks and is fast.
mod_speedyfeed [for Apache 2] allows you to only send new entries in your Atom feeds down to the clients. The client program adds a few HTTP headers (a If-Modified-Since to tell you what the last time they got was and an A-IM that indicates you support the 'feed' IM) and things just magically work.