Kahoot! is probably the best in-class game out there for full classroom participation. You can use it to break the ice, introduce topics (with blind Kahoots) or reinforce learning. Make sure to search the already-made games to see if you can find one that suits your needs before creating a new one.
Create your own Jeopardy-style game or search for a game that fits your needs. Very simple and easy to use!
This site has a lot of great ideas for games, many of which need no props or printed materials. But there are a lot of printable materials as well, and the discussion question cards are especially useful for playing with blank board games.
Quizizz is a great option for independent study since there is no team option and the teacher doesn't need to display a screen. Students are asked a multiple choice question that is then followed my a funny meme. You can set the games as homework so students can play at any time.
Quizlet Live is great for big classes since it randomly generates teams. It uses vocabulary sets that you either make or find on Quizlet.
BAAM is a powerpoint game that is great to use when you want to make winning not about ability, but about luck. You can use this template to create your own. It is not an online game, however, so you can't search for a ready-made one.