L2 protocols

Trust protocols (not always auditable)

• Full custody (Coinbase)
• 2-of-3 arbitration / DLC
• Threshold multisig (ecash, Liquid)
• Off-chain peg-out tx (statechains)
• Collateralized custody

Payment channels

• Lightning Network
• Pools/factories

"SPV" two-way peg protocols

• Sidechains paper
• Drivechains
• Fraud proofs
• Softchains
• Availability sampling

On-chain token protocols

• op_return / on-chain (Omni)
• Off-chain validation (RGB)

Perpetual one-way peg

• Vanilla spacechains
• Fee-burning "two-way" peg

SNARK protocols

• Full on-chain data (zk-sync)
• Bit field data only (zendoo)

Trusted hardware

• Single device (opendime)
• Remote attestation (teechan)

Has potential (may not work)

• Probabilistic payments
• Plasma Cash

Other related topics

• Merged-mining
• Blind merged-mining
• Stakechains
• Proof of publication