Better Gmail 2
Gmail may be the world's best Web-based mail service, but that doesn't mean it's perfect. It has one big drawback: You can't really customize it. That's where Better Gmail 2 comes in. It's a Firefox add-in that offers many kinds of customizations, including letting you decide whether and how to show "Blind CCs," what information the sidebar should display and leave out such as hiding the spam count, how to display your messages, and more.
One of my favorite features is also one of the more subtle ones. When you have an attachment, it won't merely display the generic attachment icon, but will display an icon that identifies the attachment, such as the Word icon for Word documents. In addition, it shows right at the top of the Firefox tab not just the total number of messages you have, but the number that you haven't read.
Better Gmail 2 is an update of the original Better Gmail extension, written to work with the overhaul that Gmail was given after the original extension was written. So, for example, it kills its skins feature because Gmail itself now has skins capability. It's also fixed a number of problems that the original Better Gmail extension had when working with the new Gmail interface.
Note: This link takes you to the author's site, where you can download the latest version of this Firefox-only software.