I agree with Dave. In Glasgow we started off with a couple of "SmartGroups" mailing lists. We have now moved to using two no2id-hosted lists, which are far more reliable, offer much greater flexibility and don't require us to ask people to register with a third party. The only downside of the no2id-hosted lists is that we don't receive bounces when sending invitations to non-existant e-addresses, so we don't know which scrawls (provided on petitions) to spend time on trying to reinterpet.
We check email addesses before they go on to the lists by sending an initial "Thanks for visiting the stall" email directly to each one (using BCC, so none of the recipients can see each other's email address). Any bounces from this can be used to correct/eliminate bad addresses before they get added to the list.
The reason for having two lists is so that we can have one (campaign list) for discussion among committed supporters and another (announcement list) for contacting people who want to be kept informed but do not want to be swamped by e-mails. It is very noticeable that many people drop off the announcement list if it is often used so I try very hard to minimise the number of messages sent to it, using it only when necessary. Although it is tempting to tell everyone everything that is going on, it should be assumed that anyone who is interested will subscribe to the national newsletter (which definitely gets widely read and gets results!)
Agree on all points. Cambridge also uses two lists in exactly this way, and the arrangement works well for us. Our "announcement" list helped us find 25 people to sign a joint letter to the paper which got us on the front page, a mob of people for a lunchtime photo opportunity, attendees for "Question Time" in Cambridge, and several other one-off events. However, we too find that if it's over-used, people drop off, so I try to limit announcements to one per fortnight at most.
On the other hand, the activists' discission list can get quite busy.
It is very difficult to estimate how useful websites are. They don't necessarily get many hits, but they sometimes prove quite useful. I wouldn't recommend spending a great deal of effort or money on creating one, but something simple can usually be knocked-up and maintained fairly easily and you may well find that there are local people who would be happy to volunteer to take on the task of further developing a website; it is, after all, a much easier way for the less-committed to contribute to the campaign than standing with a stall on a wet and windy afternoon.
Also agreed. We have a rather skeletal web site that doesn't get much maintenance, but at least lets people know we exist. I don't think we'd miss it much if it wasn't there.