Tom Clancy's Rainbow Six Siege is free to try this weekend. The Steam Page of the game reads, "if you'd like to discover the speed, strategy, and tension of Rainbow Six Siege's high-stakes counterterrorism matches for yourself, you can jump in for free on all platforms from February 2-5. The free version of Rainbow Six Siege offers access to all of the maps, modes, and Operators present in the full version, giving you the full Siege experience over the free weekend."
The game can be downloaded from Steam and be played for free until 4PM EST on February 5. And after playing, if you feel like buying it, you can grab it for half of the normal price (i.e. $24.99). All the progress you make in the trial can be carried forward to the full game.