It's back again this year:
http://walkaboutnyc.com/ - June 10th.
An open house for the internet companies & startups of NYC. Last year I had a great time doing this walking tour with friends. Highlights were the awesome offices of Etsy and Meetup. Also, seeing the hell that is the Gawker offices was also very eye-opening. They added Kickstarter this year, and I am tempted to go again to see them and some of the others I missed last year.