So the most powerful transmitter I've seen so far is 1.5 watts, and for features, I'm liking DLO's product, the TransPod. But it's not perfect. One thing I think the transmitters should have is a scan function where it will stop transmitting and and scan the available frequencies to find the cleanest one for you.
But as I look at how little power the transmitters have (1.5 watts gives you about a 30' range), I keep thinking of the larger FM Stereo transmitter kits they sell at Fry's and how cheap power inverters are. Just think of the fun on road trips when you caravan with a group of people and one car at the front can play "DJ" and then broadcast when to turn and stuff over the music. Throw in a few TalkAbouts and people can "call-in" requests.
Up the power a little more and you can relive your Pump Up The Volume fantasies. Cockring is optional.