Chats and Forums

Ars Mathematica
News and discussions on the mathematical arts.
http://www.arsmathematica.net/

The Institute of Mathematics and its Applications Discussion Forums
Discussion of mathematical issues with councillors and members of the UK professional body, open to all.
http://www.ima.org.uk/forums/

Javamath-discuss
A general mailing list for discussion about mathematical software development in Java.
http://lists.sourceforge.net/lists/listinfo/javamath-discuss

JISCmail
Mailing lists in mathematics and statistics for the UK academic community.
http://www.jiscmail.ac.uk/mailinglists/category/Maths_&_Statistics.htm

Math Forum: Public Discussions
Group topics include general math, education, geometry, software, calculus reform, research and recreations.
http://mathforum.org/kb/forumindex.jspa

Math Help Forum
A discussion forum for students and teachers of mathematics at school, college and advanced levels, including homework help for students.
http://www.mathhelpforum.com/

Math is Fun Forum
A discussion forum for K-12 math students and teachers, with a focus on learning and teaching math the fun way.
http://www.mathisfunforum.com/

Maths is Toothpaste
Mathematics is useless on its own, it only has a function when used with something else (i.e. a toothbrush). An online discussion.
http://mathsistoothpaste.blogspot.com

Maths-Education
Email forum for those working in mathematics education to share ideas and experiences, and request information.
http://lists.nottingham.ac.uk/mailman/listinfo/maths-education

Mathstore Electronic Discussions
A list of mailing lists compiled by LTSN.
http://mathstore.ac.uk/etalk/index.shtml

Undernet Math
Topica email list serving the users of the IRC channel #math on the Undernet, for discussion and challenging puzzles.
http://lists.topica.com/lists/undernet-math/

WebMath
A mailing list for those interested in mathematics on the World Wide Web.
http://west.camel.math.ca/cgi-bin/wcms/webmath.pl

Yahoo Groups : Erdos
A group for proposing problems, sharing ideas, discussing areas of research and reviewing papers and books of interest. [Requires Yahoo! membership]
http://groups.yahoo.com/group/Erdos/

Yahoo Groups : maththeorists
Discussions relating to number and mathematics philosophy.
http://groups.yahoo.com/group/maththeorists/

Yahoo Groups in Mathematics
Mailing lists and discussion groups (including former OneList and eGroups).
http://dir.groups.yahoo.com/dir/Science/Mathematics

Yahoo Groups: Math For Fun
Focuses on solving challenging math problems.
http://groups.yahoo.com/group/mathforfun/

Yahoo Groups: Theory-edge
Informal discussion of research in algorithmics, mathematics, and their applications.
http://groups.yahoo.com/group/theory-edge/