The title is the question :)
All the addresses I have seen - from the very first block http://blockexplorer.com/b/0 to the very last one http://blockexplorer.com/b/229850 all begin with "1".
Why is that? And - if it's really like that - why do we need the redundant number there and not just cut it and make the addresses 1 letter shorter?
If an address begin with "1" then it's a hash of a public key, "3" for a script hash, "5" for a private uncompressed key, etc
You can find a list of all prefixes here: https://en.bitcoin.it/wiki/List_of_address_prefixes
Because bitcoin and all of its derivatives have addresses start with their own prefixes, I can write a program that can tell between Litecoin, Namecoin, Bitcoin, and testnet addresses.
Imagine if you couldn't - lets say you see a *coin address on the website of an open-source project, and send some bitcoins to it. It turns out that it's actually a devcoin address. Of course, they could have clarified that it was a devcoin address, but that would take more than one character :)
It also reinforces the point that they're different coins.
