Great tools for the Bitcoin user!
1. Bitcoin Block Explorer
Bitcoin Block Explorer allows you to easily view information about the blocks, addresses, and transactions created by Bitcoin. It's a simple website made in plain text with no images. If your query is too complex it might not give you an answer though.
2. Blockchain Info
Blockchain Info allows you to navigate the Bitcoin blockchain, that is, the record of all transactions ever made on the Bitcoin network. It's very much like the previous Bitcoin Block Explorer site, but it shows the results to your queries in a much prettier way.
Bitcoincharts provides financial and technical data related to the Bitcoin network. It's also got links to important Bitcoin news, data on how the power of the network is distributed (Hashrate Distribution) and allows you to buy Bitcoins from people who live close to you through the site LocalBitcoins.
4. Bitcoin Watch
Macroeconomic data, technical analysis and Bitcoin market overview. Bitcoin Watch provides very complete, fast to access and useful information for those who want to know the economics behind Bitcoin. It takes much of its information from Bitcoincharts.
5. Bitcoin Clock
Bitcoin Clock is an original way to represent the state of the blocks created since Bitcoin was created. Here you can see the blocks that have been created, how long it will take until the block creation difficulty is adjusted, the remaining time for the reward a miner gets when he finds a block to drop by half, etc.
6. Bitcoin Sentiment
How will the price of one Bitcoin in United States Dollars vary six months from now? Will it be worth more than today or less? On Bitcoin Sentiment you can cast your opinion and see the combined opinions of the rest of people who visited this page, so as you can plan your Bitcoin inversions easier.
7. Bitcoin Monitor
Bitcoin Monitor visualizes the activities on the Bitcoin network: coins being moved around (transactions), recording and tamper-proofing the history of events (block creation) and exchanges with other currencies taking place (currency trade). It's a pretty informative and hypnotizing tool.