Mailman is a mailing list manager that archives messages and much more, and has a nice webbased interface.
MHonArc is a mail to html converter that will work with a large number of formats.
However, with the minimal information you gave, it's hard to recommend anything? What listserver are you using? What platform is it running on? How much control do you have over the computer running it? Are you looking for something to integrate nicely with an existing page/site, or can it stand alone?