When running a fresh client, or after not being connected for a while, the client takes some time downloading new blocks. How can you find out what is the target block number (the id of the newest publicly available block)?
Block Explorer displays the latest block.
You could also use Biteasy.com to find out.
The latest block is always visible at the top of the blocks list and it includes a date or if you need to find out via API then you can use the following API call which will return the latest one (in JSON format) since they are sorted by date (newest to oldest):