Mailman comes with a built-in web-based archiver called Pipermail, although it can be configured to use external, third party archivers.
Note that senders can control whether their own posts are
archived, on an individual per-message basis. If the posted
message has a X-No-Archive: header (regardless of
value), or a X-Archive: header with a value of
No
(case insensitive), then the message will not be
archived, although it will be treated as normal in all other
ways.