Gmail Labs is great. Started over a year ago, it means the Gmail team can roll out as many add-ons or features as they want without worrying about over-loading people. Any user can activate as many of these add-ons as they like. After a while, Google will promote the more popular labs features and give them a permanent place in Gmail. The less popular will be trashed.
Today Google is doing a nice spot of promoting and trashing. A few of the features that made the cut are the Forgotten Attachment Detector, which detects phrases like "I've attached" or "See attached" and, if there is no attachment, asks you if you meant to add an attachment before you clicked send; Search Auto-complete; and YouTube Previews, which allows you to watch linked YouTube videos right inside your email.
Unfortunately, Google has 'retired' a couple of nifty features too; IP-based Location Signature, which tells the receiver where the mail was sent from and the Random Signature feature, which automatically adds an interesting quote or signature to the bottom of your emails.
To peruse all the features available through Gmail Labs, click the Labs tab in settings.