Here's my picks:

Del.icio.us for helping me to not only organize and share bookmarks but find more great sites saved by other people like me.

www.stumbleupon.com for letting me browse funny after funny site, as deemed by the voting public. It's a great example of the practical usefulness of a bunch of people "training the machine" by giving content a thumbs up or thumbs down.

Along the same lines the veritable www.digg.com, one of my alltime favs, lets users not only share and validate content, but communicate popular and emerging ideas. I especially like the realtime user/post visualizers at Digg Labs.

In general, the technique of user tagging and weighting has really come to the rescue of conventional ontologies smashed by the tidal wave of data brought by the Internet.

I'm also going to put www.feedburner.com on my list, because it's a great implementation, but its also a great example of using and re-using RSS feeds. RSS, combined with users recording their thoughts in blogs, creates a truly organic/sustainable idea-sharing network.

And lastly, the buckaroo that finally tamed email for me: gmail.com. Search don't sort baby! ;)