Flock

Tagging, integrated into the browser: Flock is a new Firefox-based browser with interesting del.icio.us support built in: Your del.icio.us bookmarks are your favorites in Flock, and tagging is built directly into the browser interface. It also provides direct blogging tools; I am editing this post in Flock itself, not in Blogger's web interface. This is in beta, but this is already a fantastic tool and may be the killer app for tagging.