Firefox with Greasemonkey: During Gmail's latest design upgrade, the multiple selection links, including the "Unread" selector, were moved into an integrated box. Gmail fiends like Matt Kruse loved that "Unread" link, so he created a Greasemonkey script that brings them back.
The script doesn't install on Chrome at the moment, and from what we've read in the script comments, doesn't run perfectly on every setup all the time. But from our tests (Firefox 3.6 on Windows 7), it simply does what the label implies: brings back the All, None, Read, Unread and other selection buttons to the line just below the buttons. All those links are still in the new drop-down box on the left, but folks like Kruse just found that Unread link a vital part of their Gmail routine.
The "Restore 'Select' line" script is a free download, requires Firefox and the Greasemonkey engine.
Gmail - Restore "Select" line including "Unread" link [Userscripts.org]