iNaturalist is described as 'Get outside, and observe an individual organism. Pick something wild and take a clear, full frame photo. If you already have a photo of something wild, add it now. You can also use the iNaturalist mobile apps to record observations'. There are more than 10 alternatives to iNaturalist for a variety of platforms, including iPhone, iPad, Android, Android Tablet and Online / Web-based. The best alternative is Pl@ntNet, which is free. Other great apps like iNaturalist are Gardenia (Free), Leafsnap (Free), Agrio (Freemium) and PlantSnap (Freemium).
Pl@ntNet is an image sharing and retrieval application for the identification of plants. It is developed by scientists from four French research organisations (Cirad, INRA, Inria and IRD), and the Tela Botanica network, with the financial support of Agropolis fondation.
This simple site gives you access into the incredible Trefle plant database. Search all the plants of the world (almost!) over multiple criteria, get up-to-date information, taxonomical references, images and more. Now includes image recognition.
Plantifier is a free community based plant image recognition app. 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!