1). Lightning Network: The Lightning Network, which was introduced in 2018, leverages state channels to allow microtransactions on Bitcoin. By completing several transactions off-chain and settling the opening and closing balances on the main blockchain, it enables quick and inexpensive transactions.

2). Rootstock: Rootstock introduced smart contracts on the Bitcoin blockchain by functioning as a sidechain. Sending Bitcoin to the Rootstock network enables users to store it in their RSK wallet as locked-up smart Bitcoin (RBTC), which facilitates faster and less expensive transactions.

3). Liquid Network: Liquid Network is a Bitcoin Layer 2 sidechain that enables its users to transfer their bitcoins back and forth. BTC is changed at a 1:1 ratio into Liquid BTC (L-BTC) upon transmission to the Liquid Network