There are many apps available and you may already have some favourites. What is listed here are a few that are FREE and received good reviews.
And remember that Google Images allows you to upload photos for identification or drag & drop an image in to retrieve similar images in your results.
GARDEN COMPASS
Garden Compass (gardencompass.com). Simply submit a photo of the specimen and an advisory board of expert green thumbs will play “plant detective" until they’ve made a positive ID, a process that, in most cases, takes less than 24 hours.
GROWIT!
A social gardening app that allows you to share your plant photos, see what other members in your area are planting and also get plant information.
ID WEEDS
A free app offered by the University of Missouri, Division of Plant Sciences allows hand-held device users to identify hundreds of different weeds using a variety of search characteristics.