Mac only: If you're looking for a lighter weight alternative to Firefox or Safari, you can't go wrong with Camino, Mozilla's native Mac browser. The new version includes built-in Flash blocking, Safe Browsing, and a new and improved UI.
For those of you that haven't used Camino, it's an open-source, Mac-native browser that runs on the same Gecko rendering engine as Firefox. It's not quite as feature-packed as Firefox, but it's fast—and still contains the essentials, like session saving and customisation of tab behaviour. It integrates with OS X much better than Firefox, too, including Applescript, Bonjour, and keychain support. And, of course, there is a substantial plugin development community (though much smaller than that of Firefox) over at PimpMyCamino.
After spending many months in beta, Camino 2.0 is finally in release candidate stage, and it comes with lots of improvements. Camino's had built-in pop-up and ad blocking for awhile, but with 2.0 they've added Flash blocking to the list—complete with an exceptions feature that lets you allow Flash from certain sites (YouTube, for example). Camino 2.0 also uses Google Safe Browsing, which alerts you when you're about to enter a site that Google has identified as harmful. Finally, the UI has also been updated to look more like other Leopard or Snow-Leopard applications.
Camino is a free download, Mac only. If you're a die-hard Camino user (like this intern), let's hear your favourite bits about this handy browser in the comments.