dsynscan is a libnet/libpcap based forking TCP scanner that is useful for performing high-speed scans against hosts on your local network.
It is useful mostly as a proof of concept for a forking TCP scanner. Forking scanners have an advantage that one "thread" can focus simply on sending packets out, and another can just listen. This concept could be extended so that sending out the SYNs and listening for responses are performed on separate physical machines.
dsynscan is a toy. If you are looking for a quality port scanner, check out Fyodor's nmap instead.
You can download the source or you can view the source online.