Firefox 2.0. Session restore, new tabbed browsing features, and a spell checker for every form field. I love the new tab designs, and not just that each tab has its own close button. (Unlike Safari, the close button does not appear unless the tab is selected. Time will tell if that's really a good idea.)

I'm also trying out the del.icio.us plugin, which replaces the browser's bookmarks facility with a taggable, networked bookmark facility that uses your del.icio.us account. My biggest gripe is that the browser's local bookmark feature is completely unavailable while this plugin is installed, which means I can't bookmark work-related intranet and testing sites without sending potential company secrets to Yahoo. That said, this plugin is likely to replace my own makeshift networked link collection solution for its browser integration.