You can include HTML in the messages, and the mail archive will interpret it.
Here is one example:
And it also highlights URLS. The documentaiton for hypermail is in
http://www.eit.com/software/hypermail/hypermail.html
Pedro Szekely
USC/ISI, 4676 Admiralty Way, Marina del Rey, CA 90292
Phone: 310/822-1511, Fax: 310/823-6714
URL: http://www.isi.edu/isd/szekely.html