Firefox with Greasemonkey: Gmail Compactor replaces Gmail's standard top-left links—Compose, Inbox, Sent Mail, etc.—with clear icons, while also moving the search box underneath, saving left-hand space for links, labels, and other widgets.
Lifehacker commenter Pasha, also known as Pavel Gutin, dropped a link to the latest version of Gmail Compactor in the comments of our look at Gmail Redesigned 3.0, and for good reason. His user script offers the same kind of space-saving, interface-pretty-ing tweaks as Redesigned, just on a smaller scale. When the script works, it frees up more space in the left-hand pane for helpful items like Quick Links, labels, and whatever gadgets you can't live without.
We say "when the script works" because, well, it's finicky. Pavel himself notes at the script download site that it doesn't always play nice with other Gmail scripts, and certain Gmail Labs functions seem to disable or hinder it. My personal Gmail account has a lot of stuff enabled and didn't fly with Gmail Compactor; my wife's account, with just Offline and Quick Links enabled, worked fine. If you're a Google Apps user, you'll notice the icons work, but they might link you back to standard Gmail addresses, so it's best to wait until that very fix-able bug gets quashed.
If you find another script that Gmail Compactor does play well with and complements nicely, tell us in the links. Gmail Compactor is a free download, requires Firefox with the Greasemonkey extension installed. Thanks Jeromy!