Opera... no one actually uses that anymore lol.
Opera is my primary Web browser, it has maintained about 2% market share for a few years, and it's especially popular in Belarus and the Ukraine; it also has a reputation for being fast and light on system resources, although with the advent of extensions in Opera 11, it's just as easy to bog that browser down as Firefox or Chrome.
Opera has introduced or popularized several ideas that later made it into other browsers, like integrated content-blocking (via the URL Filter file), tabbed browsing, Speed Dial, tab grouping, and that expandable menu button that Firefox 4 blatantly copied.
It also includes several things that most browsers require extensions for, without becoming huge and bloated, like Userscript support and IRC, BitTorrent, E-Mail, and USENET clients; the same could be said for its suite of developer tools (Dragonfly), but now Firefox is the only major browser that still relies on an extension (Firebug) to provide decent dev tools.
Additionally, Opera has tended to have the lowest number and severity of security vulnerabilities, and although maintaining the means of hardening it further (like a good ad- and malware-blocking URL Filter, and the Allblocker Userscript) cannot be done automatically, and sometimes it requires the browser to be closed and then re-opened, there's a reason why about 26% of malware authors themselves use Opera (over half use Firefox, for similar reasons...they don't want to get so easily infected via their own exploits).
(On the other hand, the real weak points are now in the plugins, and almost all live exploits use patched vulnerabilities, so keeping your Java, Flash, and Adobe Reader up-to-date is more important than anything else.)