No need to PANIC!
Gmail has lots of 'little extras' to help you out.
Go to the Labs section in your account, Settings>>>Labs and search for Undo Send.
Click the 'enable' radio button and save your changes.
Now you're good to go - providing you act quickly!
The length of cancellation period can be adjusted under Settings>>General.
Have you any quick tips for gmail? Let us know about them by commenting below.