Most of these are free, but even the sites selling their games still have some for free.

jigzone.com/puzzles/daily-jigsaw

websudoku.com

gamesforthebrain.com

lumosity.com/brain-games

proprofs.com/games

sheppardsoftware.com/braingames

prevention.com/health/health/brain-games-training/brain-games

positscience.com/games-teasers

eyetricks.com/braingames

playwithyourmind.com