I think that the hallmark of a useful forum or email discussion group is that any reasonable question should be likely to get a well informed and polite response (ideally quite quickly).
The ones I tend to find most useful are:
cre8asite: aimed at "building better websites together" this site has forums (fora?) on usability;site planning and preparation;website design;website programming;graphic design;CSS, style and positioning;writing copy and content for the web;
online marketing and promotion; Business and marketing; Internet law and ethics; computer maintenance and security; Blogs, RSS and syndication; tutorials and virtual learning; measuring your success; and search engine optimisation.
london_usability is a yahoo! group (so can be used as an email group) - though recently most posts have been about usability job vacancies.
webanalytics is a busy yahoo group discussing ways of measuring traffic to your website - subscribing to this will mean you get a lot of very technical discussion emailed to you.