LiChess: Learn from your mistakes
Lichess is so awesome, it really changed my life. The features I use most:
1. The tactics trainer is incredible and fun. You get tactics ELO and the tactics stem from real games. Hence you can replay the game and see how this position arose.
2. The new tactics trainer from your own games is wonderful (but many other desktop programs had this before). 
3. The analyse game function using stockfish is wonderful. Even continuously with a few lines + arrows in the game, you get immediately what you have missed.
4. The study section is awesome. I tried to build up a complete new opening repertoire with it in parallel to the leading chessbase tool. And guess what: It's free and a nice competitor to chessbase. All study pgns can surely be exported and imported to chessbase. 
5. I really like the simul section. You find many dozen simuls a day. 25-50% of players are with 2000-2500 Elo.
6. And if you watch the lobby in the diagram mode you always find your right partner, right time, right chess variant.
7. And of course, the mobile version is also great.
Keep on guys! We love you.