1. Quick links
You can include links to commonly accessed features and bookmarks here for easy access
2. Superstars
Mails can be tagged with different colored star icons according to their importance or categories for easy visual reference. Google has provided 12 icons of different colours to tag your mails with [12 features, 12 stars????] . A useful feature IMO.
3. Picture in chat
Enabling this features will show you the pictures of anybody with whom you are chatting with them on Gtalk, provided they've uploaded a picture themselves [duhh!!]. Not too sure about its usefulness.
4. Fixed width font
Allows you to view the message in Fixed width font. Useful if viewing ASCII arts and such.
5. Mouse Gestures
Invokes mouse gestures to perform tasks like moving through messages and going back to inbox. The perfect tool encouraging laziness, IMO. Will definitely find it useful.
6. Signature Tweak
Again a useful feature that allows you to place your digital signature above the quoted text to which you are replying.
7.Random signature
It will take the feed from a quotes website and included it as a digital signature for your messages. By default, if enabled, it would receive feeds from brainyquote.com and use it as your signature. A fun feature to have, though no useful purpose served.
8. Custom date format
You can choose the format in which the dates are shown in the messages received. Convenient feature to have.
9. Muzzle (????)
Will hide your friends status indicator in Gtalk. Save screen space.
10. Old snakey
They have included the all time classic game Snake. You need to enable mouse gestures to play this. Fun thing to include.
11. Email addict
In my opinion, the most innovative feature. When enabled, it will show a link on the top right corner of your screen - Take a break. Clicking on it will make the window inactive, thereby preventing you from using your email account in any way for 15 minutes
12. Hide unread counts
Will hide the number telling how many unread message you got in the inbox. Probably a space saving move, not sure about its utility.
For some reason I was able to get the Labs option only when my account was opened in Firefox. Opera and Internet Explorer did not show me this option for some unfathomable reasons. Labs is currently available only for users who have selected the English version.
As these features are currently experimental, there are chances of they withdrawing some/all features. They have even provided a undo link on the Labs page itself incase you are unable to.
http://mail.google.com/mail/?labs=0
Clicking on this link would disable Labs in your account. Note it down. Try them out and send them feedbacks if you got any.
Godspeed