Gmail Labs are experimental features that are not quite ready to put into full production. They can be really useful!
To get to the Labs section, click on the gear icon in the upper right corner of your inbox and choose "Settings" from the menu. Then click on the "Labs" section across the top of the page.
To find a lab, scroll down through the list of popular labs or search for one in the search window. When you find one you'd like to enable, you can simply click the bubble next to enable and then click on the "Save changes" button on the top or bottom of the page. Your enabled labs will show at the top of the list.
Some labs that you might find useful are:
Sortd transforms the look and feel of your inbox and allows you to categorize your email and keep track of tasks easily. Dragging and dropping emails into categories and viewing your tasks lists helps you stay on target. Here is an example screen of a Sortd inbox:
You can click here to head on over to the Chrome Web Store and install Sortd Smart Skin for Gmail.
Boomerang for Gmail allows you to schedule a message and send it later. You can also open a message and tell Boomerang when you want to see that message again and it will pop up in your inbox again when you need it. There are a few other neat options involved with Boomerang. Click here to be taken to their website to read more.
*There are paid features in this one. You can use the free version and you get 10 message credits a month. That means 10 messages a month that you can schedule or use another Boomerang function with.