Lichess

Chess.com is the dominant commercial chess platform (150M+ accounts, $14-$49/year premium tiers, ads on free tier, video lessons, bot opponents). Lichess is open-source, donation-funded, ad-free and gives every user every feature for free — including unlimited computer analysis, all puzzle modes, all study features and Stockfish analysis at full depth. Chess.com has more polished video lessons, broader bot personalities and a larger user base; Lichess has cleaner UX, stronger analysis tools available to everyone, and the philosophical advantage of being a non-profit.

Yes — Lichess is permanently free with no premium tier, no in-app purchases, no ads. Every feature (unlimited analysis, all puzzle modes, all study tools, tournaments, broadcasts) is available to every user. The platform is funded entirely by user donations (suggested $5+/month patron tier that gives a profile badge but no functional benefits) and runs as a UK non-profit. Annual operating cost is roughly $500K covered by community donations. The Lichess team commits to keeping it free forever.

Lichess offers Stockfish analysis at unlimited depth for every user; Chess.com gates depth on premium tiers. Lichess's puzzle library is 4M+ tactical puzzles generated from real games. Both have opening explorers; Lichess uses the lichess.org master games database and a separate community games database. Chess.com has stronger structured video lessons and the GothamChess / Chess.com brand presence. For pure self-driven training (puzzles + analysis + study) Lichess is at least equal and is free. For guided lesson-based learning Chess.com's library is broader.

Partially. The PWA caches puzzles, analysis and the opening explorer for offline use — you can solve cached puzzles and analyze games with Stockfish (the WebAssembly version runs in your browser) without a network connection. Online play obviously requires connectivity. The mobile Lichess app (also free, open-source) has similar offline support. For pure offline play against the computer the native Stockfish iOS / Android apps are the standalone option.

Lichess hosts top-tier official tournaments (the 2020 Magnus Carlsen Chess Tour and many follow-ups, the Titled Tuesday parallel events, FIDE World Championship broadcasts) and is used daily by titled players including grandmasters Magnus Carlsen, Hikaru Nakamura (also active on Chess.com) and others. It is the chosen platform for open-source-aligned chess communities, university chess clubs, and tournament organizers wanting a free polished platform. Daily peak concurrent users surpass 150,000 during major events.