Lichess is the most complete chess experience, with a simple interface that focuses on the game, a massive array of game related tools and a huge, worldwide community.
The Lichess covenant:
lichess.org will always be free.
lichess.org will never contain advertisements.
lichess.org will always be open source.
Two-player games between humans
Single-player games (versus Stockfish, the most powerful open-source engine)
Support for moretime()
Resignation and draw offers
Ability to abort uninitiated games
Ability to force resignation
'Arena' style tournament feature More Info »
Time controls for chess clocks, including Fischer delay
Optional Black and White player randomization
Ability to request a player to take back a move
Support for variants: Chess960, King of the Hill, Three-check Chess, Atomic chess, Antichess, Horde chess
User-defined restriction of who can join games, based on player rating
Board editor to set up and practice positions against players or people.
Opening puzzles to learn good continuations
Coordinate trainer to train the locating of chess squares
Curated library of publicly available chess videos
PGN import and ability to load positions from FEN
Mobile app for iOS and Android
Works in any modern web browser (except Internet Explorer 8 and lesser versions), with no plugins such as Flash or Java needed
Availability in over 80 languages
Ability to toggle the overall theme of the website between black and white
Basic chess set and background theme switching
Ability to invite friends to a game via unique links created for every game
Optional username and password registration
Option for users to close their accounts