Open-source useful tools

Never use closed-source tools, for obvious reasons

Client forks

piotrnar's fork

Import/export transactions

Scripts

Pywallet

Wallet managing: import/export/delete addresses and transactions, recover *coins sent to Bitcoin addresses

Sign/verify messages and files with Bitcoin addresses

Bitcointools

bitcoin-wallet-recover

Recover wallets from formatted drives

Online

Blockexplorer.com

List of blocks and transactions

Bitcoin Charts

List of unconfirmed transaction

Bitcoin Monitor

Visualize the activities on the Bitcoin network