We've all been there: you spend half an hour writing a strongly worded letter, hit send, and realize you probably shouldn't have done that. Thankfully, Gmail Labs' Undo Send feature is there to save you from yourself. After sending an email, Gmail will wait a predefined number of seconds (5, 10, 20, or 30, configurable in Gmail's settings) before sending—at any point during which you can hit the “undo” button to take back your mistake. Even if you don't foresee needing it, it's a heck of a lot better than yanking out your Ethernet cable, so you might as well keep it around just in case.
Undo send has got to be my favorite Gmail feature. I use this at least once per week, and am so incredibly thankful every time I do. It is worth switching to gmail for this one feature alone!