Gmail Labs is a place where Gmail frequently tests new features .
You can visit http://mail.google.com/mail/#settings/labs and opt in to get those features.
There are lots of features. Some of the Gmail Labs facilities I really use often:
1. Gmail Offline – Downloads your Gmail messages, attachments to your desktop. So you view, search, reply your mails offline. Once you are online, the changes will be synced.
2. Pictures in chat – See your friends’ profile pictures when you chat with them.
3. 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. It would be nice if they would let us edit more “alarm words” like attachment, attaching in our own language.
So, what is your favourite Labs feature?