Is there a way to set a controlled delay for outgoing gmail messages? This is very useful in implementing second-thought changes to a message... Some people call this delay "cool-off", a very inspired designation...
Built-in to Gmail is the "Undo Send" lab, which allows you up to 30 seconds to cancel a message you have sent. (Of course, there's no guarantee that an experimental feature like this will ever be fully integrated into Gmail and may go away at any time.)
If you want to be able to send your messages at a later time, you'll need to use a third-party service like Boomerang, which, among its many features, is the ability to schedule outgoing messages.