Today, many of them are graduating to full feature status. The six Labs that have made the cut include Search Autocomplete; Go To Label; Forgotten Attachment Detector; YouTube Previews; Custom Label Colors; and Vacation Dates. I use all of these Labs, and am glad to learn that enough other people value them that Google has deemed the worth keeping.
Those that didn't make the cut? Muzzle; Fixed Width Font; Email Addict; Location in Signature; and Random Signature.
Google explains in a blog post, "These decisions were made based based mainly on usage, taking feature polish and your feedback into account. We've also tweaked some of the graduating features to improve them before making them default Gmail features. For example, we've combined Go To Label with Search Autocomplete, making it easier than ever for you to find what you're looking for."
Google notes that users should notice the Labs being retired will stop working over the next few days, and will eventually disappear from the Labs page entirely. Google promises that it will continue to create and introduce more Labs, and hopes that many of them will graduate, too.