Exarch is a free-software tool which provides server-side dynamically generated archiving for mailing lists.
It's a concurrent to products like MHonArc, Hypermail, Pipermail
Whereas these traditional tools generate static HTML pages, Exarch uses dynamic contents
Look here for an example (it's not refreshed, I know there are bugs, for demo only) of the archive of the python mailing list (130 MB of mbox files, 80K messages): with frames or without frames.
It is not yet available for download and I'm not looking for other people to work on it right now, I decided to write it again from scratch, I'll make it available and open for public development in a couple of months.