One thing that Stewart Butterfield did that changed the web was www.the5k.org.

http://www.the5k.org/about.php

They made people rethink what can be done with the web, and it brought out a lot of creativity.

For example: My favorite was the 2001 entry "fully functional 5k calendar".

https://web.archive.org/web/20021027194935/http://entries.th...

What was great about these is that they had functionality. It was the start of client side web-apps. I think without that contest website, a lot of these great creators wouldn't have gotten the recognition that they have now.