April 8th, 2010 | Published in Gmail (Google Mail)
Labels are more flexible than folders because a given email can have several labels but can't be in several folders at the same time. A highly requested feature for labels, though, comes from the world of folders: the ability to organize labels hierarchically.
If you think this might be useful to you, go to the Gmail Labs tab under Settings, look for "Nested Labels," enable it and click "Save." You'll then need to name your label with slashes (/) to make it the child of another. For example, let's say you wanted to create a simple hierarchy with a "Home" label, and inside it a "Family" and a "Vacation" label. Just create three labels with the following names:
You can then create "Home/Family/Kids," "Home/Pets," etc., to get something like the screenshot on the left. If you had the parent label "Home" before you don't have to create it from scratch.
You can create complex hierarchies of labels if that's the way you like to organize your mail, and you can expand/collapse labels to save space. You'll always be able to tell whether a given label contains unread messages in its collapsed child labels by looking at whether it's bold or not.
Please note that this lab doesn't play nicely with the "Hide Read Labels" lab. You might not get exactly what you expect if you have both labs enabled; for example, the collapse/expand icons won't always appear when they should.
Another highly requested feature is the ability to preview messages to get a glimpse on what they contain and maybe take immediate action without opening them.
This is exactly what "Message Sneak Peek" does. After you turn it on, right-clicking on a line in your inbox shows a preview pane with the message in it.
You can also use keyboard shortcuts for a faster sneak-peeking flow (enable keyboard shortcuts in Settings first if you haven't done so): hit 'h' to open a sneak peek card, then navigate with 'j' and 'k,' and dismiss the current card by pressing "Escape." Messages you peak at will stay unread (or it wouldn't really be a sneak peek, would it?).
Happy nesting and peeking!