6 links
Archives of Number Theory List
listserv.nodak.edu
Archives of postings to the Number Theory mailing list (NMBRTHRY).
FermatNumbers
groups.yahoo.com
A group for aggregating all the known properties of Fermat Numbers F(m)=2^2^m+1 in one place. Links to distributed search and lists. Archive available to non-members.
numbertheory
groups.yahoo.com
Mailing list covering recreational and serious aspects. [Requires free Yahoo membership.]
Primenumbers
groups.yahoo.com
Discussion list for those who enjoy prime numbers, primality testing, and closely related subjects.