I've just researched a bit over the internet, as it's a question that several customers are asking lately.
check this link:
http://www.rackaid.com/resources/how...force-attacks/
it's just based in iptables, so it's not a ready-to-go tool, but it can do some work for you.
I haven't had the oportunity to test it yet, so let us know how it works for you if you try it.