Pl@ntNet is described as 'image sharing and retrieval application for the identification of plants'. There are more than 10 alternatives to Pl@ntNet for a variety of platforms, including iPhone, iPad, Android, Android Tablet and Online / Web-based. The best alternative is iNaturalist, which is both free and Open Source. Other great apps like Pl@ntNet are Plant Identification (Freemium), Gardenia (Free), Tela Botanica (Free) and Garden Answers Plant Id (Free).
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!
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.