Etherscan

Etherscan is the most widely used block explorer for the Ethereum ecosystem, letting anyone inspect what happens on-chain: whether a transfer landed, what assets an address holds, and the code and call history of a contract.

The most common everyday use is verifying transfers. Paste a transaction hash and you see confirmations, fees and both addresses — the first place to look when someone says they sent funds but nothing has arrived.

It also provides live gas estimates, token holder distributions and contract source verification. Browsing is free and requires no account; only API access needs a registered key.