Some great points raised in this thread. I was also going to mention that Twitter uses 'no follow' links but Jordy beat me to it
I find this plugin very useful for quickly checking what sites use no follow links.
https://addons.mozilla.org/en-US/firefox/addon/5687
You would be surprised how many directories I have seen using 'no follow' links, but yet people still submit their sites (not that I’m a big fan of directory submission).
I would also like to share this plugin with everyone. The person who created this plugin got over 20,000 unique visits to his site during the contest.
http://www.contestburner.com/
This plugin is for Wordpress and allows you to run an automatic contest on your blog. Visitors earn points in the contest for every click the link gets. Not only that people are also awarded points for every comment they make on your blog (you have to approve the comment).
You can also upgrade the plugin for about $50, this gives added features. This includes comments on your Youtube videos and points when someone joins your newsletter.
I would love to see someone on this forum with a heavy traffic site run a contest using the plugin because it really is powerful.