Spaniel Message Board works with their News Server product to provide web-based access to the private groups which the news server makes available via nntp access. See What is Hypermail for more information. MHonArc is an email to HTML conversion tool, handy for putting mailing list archives on the web. Has links to web scripts that will let you search the MHonArc produced archives. NewsPortal by Florian Amrhein is a PHP based gateway.
Twig does IMAP and newsgroups.
Runs on Python. Easy Usenet lets you put their newsreader code on your site. Infinite Interchange enables you to run an NNTP server to host discussion groups, much like the Usenet Newsgroups on the Internet. By hosting newsgroups with Infinite InterChange, users from anywhere can access your server over the Internet, and join in on your discussions. You can set up multiple newsgroups, and there are built-in security options so that only people you authorize can join.