There is a great deal of activity amongst the fans, although not to the same extent as something as large as, say Star Trek. This short page details the main attractions.
Firstly, I am a member of the Guild of Fans and Disciples. The Guild (a non-profitmaking organisation) provides you with a lovely starter pack with badges, membership card, and the rules to Cripple Mister Onion. They produce a newsletter, Ramtops to Rimfall, approximately every two months. They also have made a rather excellent T-shirt.
Secondly I am a subscriber to TWK magazine. This brilliant fanzine is a goldmine of information, reviews, fan fiction and competitions. Highly reccommended.
Of course, no fan activity section would be complete without a mention of the L-space web where everything you could possibly ever want to know is contained.
I should also mention the Discworld Monthly magazine. You can subscribe to it here!