Background Send (a.k.a., the One New Gmail Labs Feature You Should Definitely Enable)
We love Gmail, but it can be a tad slow or unresponsive sometimes, and waiting for emails to send can really put a damper on your inbox-clearing momentum. Background Send is one of the coolest labs Gmail's come up with in a long time: it lets you continue on working while Gmail sends mail in the background. If it fails, Gmail will let you know and prompt you to try sending it again. Everyone should enable this feature; it makes using Gmail a whole lot nicer, and makes it act a little more like a desktop client than a web app.
Gmail's Priority Inbox is great for telling you which messages are important, but your "unimportant" messages can still be pretty overwhelming. You probably have your own filters set up, but now Gmail's SmartLabels Lab can keep common types of messages — like bulk mail, notifications or forum messages — labelled and organised in your inbox. It can automatically detect which messages are mass mailings, automatically generated, or sent from mailing lists or groups and label them accordingly, meaning you don't have to keep up those filters yourself. If you don't consider these messages spam, but still want to keep them separate from your other daily emails, this is a great lab to have around.
Unread Message Icon
It's been a part of our own Gina Trapani's Better Gmail extension for awhile, but Google's finally brought unread count badges to the Gmail favicon. After enabling Unread Message Icon in labs, any Gmail tab you have open should show the number of unread messages in its favicon, so you'll see whenever you have a new message.
If you cycle through a lot of messages at once, it's probably really annoying that Gmail takes you back to the inbox whenever you delete, archive or mute a conversation. The Auto-Advance Lab lets you choose what Gmail does in this situation, so you can go straight to the next (or previous) email whenever you delete or archive a message. It's small, but a good time saver and a fix for a pretty big annoyance.
Preview External Services in Messages
Gmail has quite a few labs that let you preview things like videos, documents, voicemails and images in emails if they're sent from certain services. For example, if one of your contacts sends you a document, the Google Docs Preview Lab will let you open it up right in Google Docs. Similarly, if someone sends you a message with an address in it, the Google Maps Preview Lab will automatically show you that address on a map. There are also preview Labs for Picasa and Flickr if you or your contacts use those services.
Labels are incredibly useful for organising your messages, but there comes a point where you have so many that you need to organise your labels. Nested Labels is a great little feature that lets you create labels in a hierarchy — for example, separating mail into "work" and "personal", and then further separating the personal category into "family" and "friends".
If you use Google Docs or Google Sites, Apps Search is a great Lab that extends Gmail's search capabilities to those two apps. That way, when you search for something in Gmail, it'll also bring up matching search results from Docs and Sites below the Gmail ones. If you're a heavy Docs user, you'll probably want to check out the Create a Document and Docs Gadget Labs.
Refresh POP Accounts
Gmail has a nice feature that lets you consolidate multiple email addresses into one email inbox, but it only refreshes them when it deems necessary, which can be annoying if you know someone's sent you an email that you want to read. The Refresh POP Accounts Lab adds a button to your inbox that refreshes POP accounts on demand, which is a lot easier than refreshing it from Gmail's settings or signing into your other email account.
Sender Time Zone
Time zones are always a pain to figure out, so if you regularly contact folks on the other side of the country, the Sender Time Zone Lab is probably a good one to enable. It will detect your contacts' local time zone and tell you what the local time is whenever you open an email from them. It'll even give you a little phone icon that turns red when it's after 9pm there, so you know it probably isn't a good time to give them a call.
Video Chat Enhancements
Google Chat is one of our favourite video chat platforms, and the Video Chat Enhancements Lab makes it awesome. This lab adds the latest and greatest features to Google Video Chat, which currently includes higher resolution and bigger windows, with more to come in the future. If you use Google Video Chat, there's no reason not to enable this one.
Those are 10 more of our favourite Gmail Labs, but there are countless good ones over in Gmail's settings. So head over to the Labs area and look through them yourself — you're bound to find a few you didn't know were there and are perfect for your workflow.