[The Real Slim Shady]

Well, im not in the know on altering packets in real time... nor am i much at game cheating so im not sure in what other scenarios you would necessarily need that ability in.

That said, packet creation is very useful knowledge. Personally i use hping3 ( http://www.hping.org/ ). Very cool tool for network/computer hacking as you can craft custom packets to test whats allowed through a nat/firewall/etc. eg spoof source ip/ports, headers, specify destination ports. Supports pretty much the whole IP stack i believe (TCP/UDP/ICMP to say the least). Its been a couple years since ive used it, but i doubt anything much better has come along.