Playing with Network Stack

Greetings, I hoping someone here can help point me in the right direction. I had an idea for project, but I lack some of the know how. I was wonder if it might be possible to have a kernel module or an application that can inspect and mangle a packet before it gets routed? I don't want to just sniff the traffic. I want to be able to look at the packet and then hand it back to the network stack for processing. If so is there some reading or example code I could look at? Thanks in advance, -James