Some sites like http://www.bitcoinplus.com/generate allow that.
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You should have a look at this question about web miners.
However, it might not be as profitable as you think. Considering that most people have very poor GPUs. At the moment if you'd have someone with a decent 200MHash/s card on your website for 24h, you'd basically earn 3 cents. You can check how much you could earn by visiting my calculator. All in all, you're better with ads.