That's cool. According to Google Mail software engineer Manu Cornet, you can now add nested labels to your Gmail. Posting to the official GMail blog Cornet says that labels are more flexible than folders as "a given email can have several labels but can't be in several folders at the same time". Which is why the GMail Labs folks have been listening to user requests, one of which was for the ability to organise those labels hierarchically. You can find the new Nested Labels option in the Gmail Labs tab under Settings.
Cornet adds "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".
And talking of much requested features which have made it into the Labs for users to play with, how about the ability to preview messages without opening them and keeping them marked as unread? Yep, Google has added a Message Sneak Peak feature that enables a right click preview pane view of messages from the inbox. It even works with a keyboard shortcut of 'h' to open the sneak peak card, which can then be navigated using the j and k keys.