To enable 'Undo Send':
1. Log in to Gmail and go to Google Labs. If you've never gone to Labs before, click the word "more" in the very top left corner, then scroll down and click "even more."
2. In the column on the right, click "Labs." It's next to the icon of a beaker filled with green stuff.
3. Click "Gmail Labs" in the column on the right.
4. Scroll down almost all the way to the bottom until you see "Undo Send." Click "enable" and the feature is now on. Then scroll the rest of the way down and look in the bottom left corner for the "Save Changes" box. Click it.
5. Now, go back to the main Gmail page and click "Settings" in the top right. You should also see your green Labs beaker icon there now -- this will let you go straight to Labs from now on.
6. Scroll down to "Undo Send" -- it should be right above "My Picture." Your default should be set to 10 seconds. But you can use the drop-down bar to stretch that to 30 seconds.
7. Scroll down and hit "Save Changes."
8. To undo an email, just look for the box at the top of the screen that will have the words "Your message has been sent." After that, you should see the "Undo" option. Click that and you'll be sent back to the e-mail's draft form, where you'll have 30 seconds to edit or delete it before it goes out.
For all of us that have hit send and then had that "oh crap" moment when we realize we sent it to the wrong group, wrong person, or included a glaring spelling ommission follow these instructions to enable GMails Undo Send feature.