The web mail monster that is Gmail has just had a few more limbs grafted onto its torso in the shape of two features called Nested Labels and Message Sneak Peeks.
The former is a way to organise you message labels into hierarchical structures like folder trees, such that one label can be a sub-set of other labels. For those who have no idea what we're talking about, labels are essentially message tags attached to your e-mail. So, they might be tagged as Home, Work, Family, Shopping or whatever you'd like to create.
Some messages, of course, fall into a number of categories, so can have more than one label associated with them. Now though, you can tuck some of these label categories inside others by turning on this feature in Google Labs, but do be warned that if you're already trying out the "Hide Read Labels" tester, it's going to cause some chaos.
The second addition is somewhat simpler and is a way of seeing what's in a message without actually opening it. With Message Sneak Peeks, you can right click on one in your list in the inbox and a small window will appear with a good portion of the e-mail's contents displayed. All that without having to navigate away from your inbox only to find it's the wrong e-mail or something you really don't need to read.
As both these features are testers, you'll need to have Google Labs turned on which involves heading over to "Settings" and heading to the Labs tab. Have fun.