One of the best is MARK•R, which uses AI to offer real-time information, summaries and explanations for selected text. That can help you better understand web pages through interactive questioning and personalized explanations, making it good for learning, research and content creation. It's a Chrome extension, with free and paid versions depending on your needs.
Another strong contender is Buffup, a ChatGPT-based browser extension that acts like your own AI assistant. It offers a variety of AI-based abilities, including intent detection, chat, reading and writing help. You can use Buffup to analyze web pages, summarize documents and even generate articles and headlines. It works on Chrome and Edge browsers, with a variety of pricing tiers you can pick from.
If you want something more basic, a summary and explanation tool, BLUF is worth a look. It's got an "Ask" feature to narrow your query, "Summarize" to focus on the most important points, and "Explain" to try to break down complex ideas. BLUF is a Chrome and Firefox extension that runs your query on OpenAI without storing any content, so it's private and efficient.