Brain Games - Boost memory power with free brain games
WeKAb - Connecting Knowledge
Dualnback.works - Dual N-back is a specific brain training exercise, that's been shown by research - it might improve fluid intelligence.
Newton Virtual - Learn and enjoy
Pixelogic - Daily Picross - Nonogram logic game with 5000+ puzzles and a daily puzzle
Crehana - Crehana is an on-demand learning platform.