http://www.phpwebscripts.com/
The script is $80 which although not free makes it far cheaper than many. However not only is the script good (and i mean really good), the guy who writes it has gotta be one of the most switched on and helpful around. If you're currently using Goss Threads script (non-SQL version) dump it and get this. You can even import the exisiting database straight into it. The blacklist feature though (amonsgt many, many others) is a life saver.
This is one of the only times I've plugged something for no money but as it's the best $80 I've spent in a long time I thought the guy deserved it.
Gotta stress the script has nothing to do with me. I just know there's a lot of overpriced crap on the market and a lot of people run with the Goss Threads script. I've been looking for a good alternative at a reasonable price for a while and finally found it so though others may find it handy.