Duolingo breaks language study into lessons of a few minutes, with instant feedback, a streak counter and a leaderboard. More than forty languages, the whole course free to use, progress synced between web and phone, and conversation practice on the paid tier.

A non-profit learning service that costs nothing to use. Courses run from preschool numeracy to early university mathematics, science, grammar, history and financial literacy, with practice built around mastery and dashboards for parents and teachers. More than fifty languages, no advertising.

One prompt box asks how it can help with your research, five starters sit beneath it, and behind them a chain of tools: search in three depths, a review with a published benchmark, chat over a PDF, drafting, diagrams, citations and a detector.

Ask a yes-or-no research question and a meter shows how far the published evidence agrees. Deep search builds the strategy for you: expanding keywords, hunting opposing claims, walking the citation graph across 220 million peer-reviewed papers.

Search 138 million papers and 545,000 clinical trials, then have the agent assemble a report, a table or a slide deck in which every claim carries a citation back to the sentence it came from.

Moyu Danci is a vocabulary tool for people who study in the gaps of a working day: a word window that fades into the background, a key that hides it, and a review schedule that decides what comes back and when.