Gmail labs is one of the best things I discovered in different mail services. It allows you to do lots of things. After logging in Gmail, just click at the "lab" tab in settings or click on the "lab" icon just after your email ID on top-right corner of your browser window and you can see the numerous features offered to you by gmail. These google guys always do out-of-the-box things...

I personally liked some of the features provided by Gmail labs:

1) Signature tweaks: I personally liked this one. Places your signature before the quoted text in a reply, and removes the "--" line that appears before signatures. But beware, you can't use this and the "Random signature" Labs feature at the same time.

2) Quick Links: Bookmarks common Gmail views. it adds a box to the left column that gives you 1-click access to any bookmarkable URL (mails) in Gmail. You can use it for saving frequent searches, important individual messages, and more.

3) Superstars: Adds new star designs to better differentiate important email. After enabling Superstars, simply click on the star icon repeatedly to rotate through new star options. you can also choose which icons you wish to use in the "General" Settings page.

4) Email Addict: Lets you take a break from email and chat by blocking the screen for fifteen minutes and making you invisible in chat.