Google went live with Gmail Labs, experimental features for web mail client Gmail which you can choose to enable one by one.
Gmail Labs are 13 new optional features that you can try out on your Gmail account. To get them go to your settings and click the Labs button.
Superstars
Now you can flag, star, and otherwise mark messages using up to 12 different colored and shaped icons. When you enable Superstars, you get a new section in the General settings area, which looks like this.
Click on the star button on a message repeatedly to cycle through all the Superstar choices. Looks like Superstars is more a visual indicator than anything; you can't search by all the messages superstarred the check, for example.
Quick Links
Basically a souped-up version of the Gmail Saved Searches Greasemonkey user script, Quick Links adds a module to the 
Gmail sidebar where you can store links to searches, views, and even individual messages. Go to the view you want—like all messages that have PDF file attachments using the has:attachment PDF Gmail search—then click "Add Quick Link" to save what's essentially a Gmail view or message bookmark there.
Email Addict
The Email Addict feature adds a "Take a Break" link to the top of your Gmail inbox, that looks like this:
When you click it, you get 14 minutes off from dealing with email:
Snakey
Well, perhaps something to surprise your friend, though not useful (or a terrific novel game) in itself; enable this, then enable shortcuts, and press “&” to bring up a game of snakes.
Custom Keyboard Shortcuts
Just as you'd expect from the name, the custom keyboard shortcuts feature lets you map Gmail actions to the keys of your choice.
Random Signature
Add a random quote to your email pulled from an RSS feed by enabling Random Signature. By default, Random Signature uses a feed from BrainyQuote.com, but you can set it to the feed of your choice in the signature settings.
Fixed width font
Enabling this will trigger a new entry in the “Reply” menu that lets you switch to a non-proportional font.
Signature tweaks
Places your signature before the quoted text in a reply, and removes the "--" line that appears before signatures. Can't use this and the "Random signature" Labs feature at the same time.
Custom date formats
Adds options to the general settings page allowing the date and time format to be changed independent of language. For example, you can use a 24-hour clock (14:57) or show dates with the day first (31/12/07).
Muzzle
Conserves screen real estate by hiding your friends' status messages.
Not much to be said about this one. It's a small change and I don't see much use for it. If I want to save screen real estate I can just close the whole box and be done with it.
Hide Unread Counts
Hides the unread counts for inbox, labels, etc.
I don't see much use for this one either. Maybe it is useful for somebody who is stressed out by seeing 300 unread emails in his Inbox.
Pictures in chat
See your friends' profile pictures when you chat with them.
It's helpful if you have more than one chat window open.
Mouse gestures
When you enable this feature you can hold the right mouse button and then move the mouse upwards to jump to the inbox, from any place. Moving left-ways goes to a previous conversation, moving to the right goes to the next.
Summary
Gmail Labs is going to be nice way to test new Gmail features before introducing them to a bigger userbase. Google is serious about it, they are hiring people to write such widgets. And, as a user you can give feedback on them.
There is no support for Google Apps accounts...
I was expecting that GMail Labs might actually offer some genuinely interesting enhancements to the email experience. Instead, what’s on offer is a mish-mash of tiny incremental gadgets to tweak very specific aspects of GMail. Nothing in the way of more fundamental or experimental features. (And nothing that even attempts to fix any of the long-standing problems with GMail like the automatic adding of all email correspondents to the address book, or the inability to add inline images).
I can only hope that things will get more interesting over time.