Rezoning cases are a big deal in my hometown. Yours, too, I'd guess. We've got another one underway, so I wrote a little primer at my personal blog for community organizing online. I'm posting it here, too -- not because you're particularly interested in what goes on at the corner of North Elm and Cornwallis in Greensboro, NC, but because this kind of stuff will have an impact on your marketing, site selection, and so on.
Let's say you are concerned about a rezoning case and want to use the internet to better organize opposition to the plan.
Don't expect miracles, and don't expect the net to replace other forms of organizing. Leverage existing groups -- neighborhood associations, social networks, religious organizations, etc -- when possible.
Build an email list. Collect email addresses at every meeting, and ask people to supply addresses of friends and neighbors. Use email to keep people informed of key developments, meetings, etc.
Start a blog. A free service like Blogger will do just fine. Choose a simple, meaningful name for the site. Post news and updates to the blog -- meeting times, press coverage, photos of the site, etc. Put new content on the blog regularly. Remember, "blog" is a funny word, but it's just a newsletter that you can create and distribute for free, and update at any time.
Use email to publicize the blog -- to group members, and the media. Don't spam people by emailing about every blog entry, just let them know the thing is there and let them know when something big has been posted (e.g., "click this link to see the zoning staff recommendation").
Walk the neighborhoods. Assign block captains to speak to their neighbors. Collect email addresses in the process. Use the phone. Use the blog and email to urge people to take action, e.g., calling public officials, writing letters to the paper.
Keep your eye on the prize. In Greensboro, the zoning meeting is just the first step, the big battle is at the City Council level. Use email and the blog to explain this, and to get people to the right places at the right times.
For a protracted battle, consider building a wiki that describes the situation. Link it from the blog. You probably don't need Twitter, although tweets from a given meeting might be entertaining.
Here's a column that touches on the subject.