Hop Protocol is a Rollup token bridge, and Hop Exchange is a front-end application created on this bridge system.

Principle: Hop relies on market makers called Bonders, who earn fees by providing liquidity to others; this automatic market maker (AMM) style allows Hop users to quickly send cross-chain transactions. This means that people who provide liquidity to jump have a chance to earn money!

Connext is an interoperability protocol, and its team built Nxtp, a system tailored for cross-chain transmission.

Principle: "You can use [Nxtp] cross-chain and/or Rollup to send valuable transaction or call data. Unlike most other interoperability systems, Connext does not need to introduce any new trust assumptions or external validators to achieve this a little.

Across Protocol is a new L2-L2 bridge protocol. Currently, the project supports Arbitrum-Ethereum transactions.

Principle: "The bridge function works by incentivizing the repeater to provide users on L1 with short-term loans. These loans are repaid from the liquidity pool on L1 two hours later. When the funds arrive from the L2 transaction, the pool is renewed Filled. These transactions are guaranteed by UMA's Optimistic Oracle.