The best free tag board I have found for my websites
Wilson's Almanac has a Tagger tag board in two places:
http://www.wilsonsalmanac.com/corrigenda.html
and here at the Blogmanac.
This free, self-hosted, ad-free Luxury Edition tag board
is designed by Brian Benzinger of venture nine solutions
http://venturenine.com/
It is excellent, and so is Brian's support.
This is an unsolicited endorsement.