Firefox may take the cake as far as extensions, plugins and the kitchen sink goes, but OS X's Safari browser has its own plugin following that's hot on Firefox's trail: PimpMySafari.com.From dev tools that can copy the HTML of a whole site with a single hotkey, bookmark managers and browsing scratchpads, this quickly expanding collection of Safari plugins, helpers and accessories has everything an OS X web surfer or developer could need.
Be sure to check out my personal favorite: Concierge. It's a "bookmark assistant" who's main appeal for me is a super-handy side drawer that can act as a browsing scratchpad. Great for doing research or holding onto some temporary links without having to actually bookmark and organize them.
