Here are my favorite experimental Gmail features, although I haven’t used all of them yet:
Conserves screen real estate by hiding your friends’ status messages.
Google strives to make the world’s information useful. Mail you send late night on the weekends may be useful but you may regret it the next morning. Solve some simple math problems and you’re good to go. Otherwise, get a good night’s sleep and try again in the morning. After enabling this feature, you can adjust the schedule in the “General” settings page.
Forgotten Attachment Detector
Prevents you from accidentally sending messages without the relevant attachments. Prompts you if you mention attaching a file, but forgot to do so.
Oops, hit “Send” too soon? Stop messages from being sent for a few seconds after hitting the send button.