This site, Forex, tracks the major currencies. I've been following it for years, but lately have placed a fair-sized investment in the euro and so now I check the charts daily. (In order to see charts of the various currencies "against" each other, click on the countries' flags in the center of the frame.) The euro popped through an historic high ($1.36 dollars per euro) a few days ago and appears to have an upper limit constrained only by trade and governmental sanity returning to the US--and hence no barrier at all.
If you've got some spare dollars lying around, you might want to climb on the bandwagon.