Google "currency ETF" and you'll find links to news stories about Rydex's NYSE-traded currency funds. Each fund holds nothing but foreign currency. Wisdom Tree apparently just filed with the SEC for approval offer some as similar funds.
You'll need to pick the currency or currencies you think will go up. Each fund has just one currency, i.e. Euros, Canadian dollars, Swiss francs, etc.
I haven't invested this way, so I can't say much pro or con. Like most on the forum with an allocation to international mutual fund holdings, I'll likely get some gains in those funds if there continues to be a sinking U.S. dollar. While I think it's likely, I'm not planning to play around with my asset allocation in an attempt to pick the currency that will provide a few extra points of return.