ettercap is a great tool for that. i assume you want to do arp poisoning, in which caase you can do that manually too by captureing the packets using wireshark, editing it, and sending it out using file2cable (i think thats what its called). i would suggest the second method as it will give you a better understanding of what goes on behind the tool. now if you wanna act as a mitm between a client and an ap, id check out airbase. its still in development but its a hell of a tool if can understand how to use it.