GamePass

As an introductory offer GamePass is giving away free 30 day memberships, which comes with one free game of your choice from over 300 (mostly puzzle games, but many other options as well). High quality games.

Free trial period, then pay to continue playing.

Multris

Multiplayer Tetris.

Free, with option to pay for additional features.

Puzzle Pirates

Pirates can wander around on land and sail the thirty-seven seas with their crew. When your Pirate sails, or swordfights, or navigates, the appropriate Puzzle game is launched. Good Puzzling thereby brings victories and accrues great fortunes to you and your fellow Pirates.

Free trial period, then pay to continue playing.

Quadra

"An addictive action puzzle game with single player and multiplayer capabilities (Internet or LAN)."

Free to play indefinitely.

TetriNET 2

Multiplayer Tetris.

Free to play indefinitely.