In general, the coexistence of floating currencies and free flows of capital appears to create great instability with tsunami waves of cash flowing hither and tither in response to some crisis or other.  A Tobin tax on capital/flows/currency exchanges  as well as onshares/cdos etc. purchases would have the effect of dampening such huge speculative flows whilst not ultimately restricting real long term investments made on rational grounds.  

It would also have the effect of raising huge revenues as a form of "ground rent" payable in return for the huge privileges the global financial system confers on the major players and ultimately do a lot to ease public deficits.  This is an area where the EU and US under Obama could really take a lead.  It is not complex to administer, doesn't require a huge bureaucracy, doesn't ultimately create barriers to real trade, and provides a mechanism for those who benefit most from the system to pay for its maintenance.

notes from no w here