Plantifier is a free crowdsourced plant recognition tool. It allows you to upload a photo of an unknown plant. The users of the MyGarden.org community behind this free tool will try to recognise the plant photo as soon as possible and give you an answer to your question!
Also try out our web based version via http://www.plantifier.com
Already more than 200.000 people have uploaded a photo of an unknown plant for recognition !