I love Alley Cat Allies. We first got involved with animal issues back in 1995 as the result of a feral cat colony in our apartment building's parking lot. We received no support or encouragement from our local nonprofit shelter nor our municipal shelter. Fortunately, we tapped into a group of "underground" cat rescuers who gave us advice and helped with placement (five of those placements are lounging around in the house as I write!)
The one organized group we encountered was Alley Cat Allies and they are still growing. They are a leader in the field of managing feral cat colonies and rescue work.
I see that they now have an anti-cruelty web page and I want to encourage you to check out the group's site and this useful page.
Photo by dawnkua.