Ask before sending an SMS or Mail message. No more accidentally sent incomplete messages! Very likely this happened to you at least once. This tweak helps you avoiding that. If you tap the 'Send' button in Messages or Mail, it will display a confirmation dialog asking you if you really want to send the message. For Mail, it also has an option to remind you if you didn't type in a subject.
This tweak directly integrates into Messages and Mail, and is configurable via Preferences.
Note to iOS 5 users: The default MobileMail.app now has a check for an empty subject. If you install AskToSend, this message will be replaced by the AskToSend (customizable!) missing subject message. However the check for missing subject is _not_ enabled by default, so make sure to check out Preferences.app -> AskToSend.
Note to iRealSMS users: iRealSMS has its own option for sending confirmation, so use that instead. The AskToSend Mail confirmation feature is still working apart from that
You might also like to check out my tweak PlayAwake which allows setting iPod Music as alarm sound in the Clock app.