It's everywhere. Lobby games, tournaments, simuls, and the analysis board; Crazyhouse is a first-class variant on lichess.org.
What is crazyhouse?
It's a very exciting chess variant. In a crazyhouse game, either player can introduce a captured piece back onto the chessboard as their own. This is called dropping a piece.
In this example, White has two previously captured Black pieces in their pocket, a knight and a bishop. These pieces were captured by White earlier in the game. White can check the Black king by dropping a bishop on c3!
In turn Black can block the check by dropping a knight on e5!
To be honest, this was quite tough to implement, but also fun and totally worth it. The main challenge being that crazyhouse changes the very definition of what a move is; since now it can also be a drop. This change to a key concept of lichess required some rewiring of the program foundations.
Please don't ask me for bughouse. Lichess is a game for one board and two players. Now, someone should totally build a new bughouse website/app. It's impossible to integrate into lichess because it's radically unlike chess, structurally speaking.