– Sonic & All-Stars Racing Transformed (often $5 during sales, including the Humble one https://www.humblebundle.com/store/p/sonicallstarsracingtransformed_storefront )
– Rocket League (cross-play with PlayStation 4 users and currently on sale https://www.humblebundle.com/store/p/rocketleague_storefront )
– Counter-Strike: Global Offensive
– Ultimate Tic-Tac-Toe ( http://store.steampowered.com/app/360870/ ), which apparently works primarily through invites anyway ( http://steamcommunity.com/app/360870/discussions/0/485623406962617943/ )
– Pokémon battles if any of you guys have and still play X, Y, Omega Ruby, or Alpha Sapphire
– Burnout Paradise: The Ultimate Box if you guys have that on Steam from when it was in a Humble bundle
– DOTA 2!!!!!! >:D