One of the cool things about Gmail is that it has so many Labs projects . You can text people, you can insert images, you can easily translate emails, you can optimize Gmail for the wide-screen, and much much more!
1. SMS in Chat & SMS in Chat gadget
Although this one is technically two different lab projects, when you combine the two you can have a really nice experience chatting with someone over text messaging.
Once you’ve enabled these two projects, you can just enter a phone number and chat:
The only downside to this is the receiving person gets a message from a number starting with 406 and a one-time message about what the Gmail chat service is.
2. Undo Send
Have you ever sent an email to someone that you really regretted? Save yourself the pain and undo the send! Although it won’t actually delete already-sent emails off other people’s accounts, Undo Send will keep every email you send unsent for up to half a minute.
After enabling the feature and sending an email, you’ll be told that the message has been sent–and that you can cancel it.
3. Insert Images
We’ll file this one under the heading of “no duh”. Gmail is the last major webmail provide to have no official, non-experimental way to insert images into your emails. After enabling this, you’ll be good to go and on your way to success!
If you set up lots of POP3 accounts in Gmail, then you probably know that the only way to check these is to either wait or to go to Settings>Accounts>Refresh. That’s too much work, isn’t it?! Enable this to have a single refresh button on the top of the page for all your accounts.
5. Previews in mail
Do you get lots of emails with Yelp links in them? Friends like to send you maps? Perhaps your mother finally figured out how to use Flickr/Picasa and is sending you lots of albums? Or is it just that you hate having to click through to Google Voice to listen to messages? If you are in any of those situations, the Previews in mail labs can help. It’s actually several different Labs, with one for each site, but getting previews in your emails is pretty helpful.
6. Inbox preview
Gmail takes a precious second to load. Why not make use of that second? Enable this lab to get an instant preview of what’s in your mailbox instead of a boring old loading screen.
What says :D better than ? Easily add almost a dozen pages of emoticons, from fruits to pets to just plain random stuff–with the Extra Emoji lab project. You’ll have these, spread across 11 pages:
…you know, in case you ever need to send someone a little dog. Or a cat.
8. Message Translation
Salut, dotTechie! Ça va? Comment vas-tu? Au revoir! Le lit est dans la chambre.
Now that I’ve used up my limited French vocabulary, wonder what you would do if you received an email in a foreign language you didn’t know. Would you bother reading it? Would you just send it to spam? What if it was an email from someone important and you just threw it out? Enable Message Translation to get one-click translations of most languages.
So that’s what it means!
9. Video chat enhancements
Do you regularly video chat in Gmail? Even if you do it once in a blue moon, enable Video chat enhancements, which brings:
Enables new features in Gmail voice and video chat, including higher resolution and bigger windows, with more improvements coming over time.
Impressive, no? Try it today!
10. Send & Archive
If you like to actually use Gmail’s Archive feature, then you’ll love Send & Archive. With a simple click of a button, you can both send a reply and archive the thread. Once your recipient reponds, the email will pop back into your inbox. It’s what lets me have an inbox of only 5 emails!
11. Right-side chat
After enabling Right-side chat, your chat box will move to the right side of the screen:
12. Unread Message icon
Checking your email every thirty seconds? Stop! With Firefox 4 and Chrome, you can just pin Gmail as a so-called “App Tab”. Pin Gmail and enable this to get the full, no-checking needed effect. When you get an email, the icon’s number will change:
Image from Lifehacker 
An alternative one that I use is to simply rely on tab colors. In Firefox 4, app tabs change color with new emails.
Bonus: Old Snakey
Ok, this is thirteen. But thirteen is close to twelve, isn’t it?
After enabling Old Snakey, just hit the ampersand (Shift+7) to start a fun, mind-dulling game of Snake:
Gmail labs is a great place to go mucking around. There’s all sorts of things you can find in there, and all sorts of things left to be made. The labs are a great testing ground for cool new features that one day–let’s hope–will be part of Gmail. I’m talking to you, Insert Images.
Visit the Labs  (under Settings>Labs)!