Not quite mainstream but this is the next step. Coming soon to millions of Yahoo! users, tagging! Great news. Congrats to Yahoo! on acquiring yet another cool small team!
From del.icio.us: y.ah.oo!.:
We're proud to announce that del.icio.us has joined the Yahoo! family. Together we'll continue to improve how people discover, remember and share on the Internet, with a big emphasis on the power of community. We're excited to be working with the Yahoo! Search team - they definitely get social systems and their potential to change the web. (We're also excited to be joining our fraternal twin Flickr!)