Bill Simmons started www.grantland.com for the express purpose of having a place for long-form sports pieces on the web. I've found articles on the site to be hit or miss, but I enjoy the Bill Barnwell pieces on gambling and the occassional Chuck Klosterman piece.

I've also had good experiences with the longer Wired articles I've read. The pieces on Stuxnet and this article (http://www.wired.com/magazine/2010/01/ff_hideandseek/all/1) about a spy were especially compelling.