There has always been a undo send, but its hidden and normally only good for 5or 10 sec.  Now you can set it for as long as 30 seconds.  Still not that much time, but the difference between 10 sec and 30 sec to undo an email can be the difference between realizing your error and realizing its too late.

Go to Google labs and look for “Undo Send”

Enable the widget.  Then go to your mail settings and ‘Undo Send’ should appear in your widget list.  Change it to 30 seconds and there you go!

There are a few other really cool widgets I found as I was looking for undo send.  I really like having my calendar appear in gmail as well as small maps showing up when an address is found.

Enjoy!