A Go light client that follows Ethereum by block headers alone, verifying state and transaction inclusion with Merkle proofs instead of holding full chain state.
Headers carry the state and transaction roots, which is enough to check any claim about the chain given a proof path, so the client can answer “is this transaction in that block” or “what is this account's balance” while storing a tiny fraction of what a full node does, and without trusting the peer that served the answer.