Merlin AI

Merlin, built by Foyer Technologies, is one of the most widely installed AI browser extensions, with over 10 million users as of 2025. It works as a persistent AI layer across your browser, accessible via keyboard shortcut or by selecting text on any page.

The core use case is contextual AI: rather than switching tabs to ChatGPT, you can invoke Merlin directly while reading an article, reviewing a document, or composing an email. It supports multiple underlying models, including GPT-4o, Claude 3 Opus, Gemini 1.5 Pro, and several others, letting users switch between them depending on the task.

Key features include page and article summarization where you highlight any text or open any article for an instant AI summary. It also handles YouTube video summarization, generating transcript-based summaries from a URL. The Gmail integration drafts and refines email replies within Gmail's compose window. You can upload PDFs or link to web pages and ask questions, build saved prompts for repeated tasks, and switch between GPT-4o, Claude, Gemini, and others within the same interface.

Merlin operates on a credit system. Free users receive a limited number of GPT-4 queries per day; paid plans starting around $14.99 per month for the Pro tier provide higher limits and access to more powerful models. Annual plans reduce the effective monthly cost.

The primary alternative is Sider AI, which offers similar browser-extension functionality. Merlin's advantage is its broader model selection and Gmail-specific features. Both are significantly cheaper than subscribing directly to multiple AI services.