The idea of a Tobin Tax has always seemed pretty obvious. Not only would it raise vast amounts of money, no-one would notice the cost. Money wouldn't wash around the world quite so uncontrollably, destabilising currencies and economies at the click of a mouse.
I tried raising the topic on Kerry McCarthy's blog a while back. Didn't get a response though... maybe the time's right to try again, now that Gordon's leaning in the same direction.