One of the best is Google Lens, a built-in feature in several Google apps that uses your phone's camera for visual search, object recognition and other tasks. It can recognize and tell you about objects, text, plants and animals, so it's good for a broad range of questions.
If you're just looking for plant identification, Pl@ntNet is a citizen science project using AI-powered image recognition. The app lets you identify plants and contribute to conservation efforts for biodiversity. It's got offline identification and community validation to improve its results.