Goldmines!   Lots of useful stuff to browse - and friendly advice and chat if you choose to join any of them.

• Allotments 4 All - 900+ members, lots of allotment and vegetable growing advice, with general friendly chat.   "Parental warning - this site is highly addictive! - and there's a sister site at Recipes4All, with ideas on using your crops!
• Allotments UK - a categorised directory of useful websites, with a forum for people interested in allotment growing.
• The Kitchen Garden Forum - run by The Kitchen Garden magazine (if you are reading this you probably already have a subscription?   If not, have a think about it).   My one grouse is that this site seems to have an appeal to people hostile to organic gardening (some contributors like provoking confrontation - it's the only site I know where organic gardeners may find themselves needlessly on the defensive).
• The BBC Gardening Message Board - Allotment Tales (a bit clunky and slow, some strange limitations, no search function, but with a gift for getting people interested - who then move on to other forums!)
• Downsizer.net - for an ethical approach to comsumption - a wide-ranging forum (including on allotments and growing), with a collection of articles; fascinating stuff on wild foods, beekeeping, bread-making, LETS - well worth a browse.
• Kitchen Gardens - Yahoo Group, with good advice on vegetables and allotments; very good on allotment politics.
• UK Allotments - Yahoo group.
• Heirloom Growers' Garden - has a very substantial American membership and experience - almost enough to stop me complaining about Leeds weather!   A wealth of information and history on vegetables.
• The Vegetable Gardening Club - vegetable growing enthusiasts, beginners and expert; includes a forum where hints and tips can be exchanged - I enjoy the Trans-Atlantic exchanges.
• Real Gardeners - friendly bunch, with wide-ranging forums, advice, links, and on-line chat.

Specifically Organic

• Organic UK - a friendly and relaxed forum for organic gardeners.   Wide-ranging and informative - follow the links back to an excellent web-site.
• Organic-L - successor to the mailing list, sharing organic gardening knowledge and problems.
• The HDRA also has a very useful forum - sadly only open to HDRA members.