FastNetMon is a very high performance DDoS detector built on top of multiple packet capture engines: NetFlow, IPFIX, sFlow and SPAN/port mirror. It could detect malicious traffic in your network and immediately block it with BGP blackhole or BGP flow spec rules. It has solid support for all top network vendors and has unlimited scalability due to flexible design. You could integrate FastNetMon into any existing network without any changes and additional hardware!
Find out more about FastNetMon here or checkout a more comprehensive list of tools over at LinkedIn here.