After all the weblog noise on tabbed browsing I'm surprised there hasn't been more fuss made of iTerm, an open source alternative to the OS X Terminal app. iTerm's tabbed interface means I can flit between SSH sessions on our local and remote Linux boxes without wasting brain cycles locating the relevant terminal window... this honestly makes a significant difference to my working day.
The application I really want to see tabbed is BBEdit. I love this editor but having more than two documents open at once makes for a very messy screen.
Update: BBEdit 8, released in September 2004, includes a 'document drawer' feature which approximates a tabbed interface.