Most online wallets will create for you a new address each time you indicate that you want to receive a transaction, or give the option of creating a new address. Some enforce this by never showing an address twice and simply handle amalgamation on the backend.

Creating a new address for each transaction is a fool-proof way of ensuring that someone has paid you, because you've given that address to only that person and no one else. There are also some privacy benefits, as using a new address makes it more difficult to trace the bitcoin trail.