What is DoFollow?

As a result of the implementation of NoFollow: link building has taken a massive hit, as many sites including Wikis, social bookmarking sites, corporate and private blogs, commenting plugins and many other venues and applets across the internet began implementing NoFollow. This made effective link building difficult for both honest people and spammers alike.
By the way, there's nothing wrong with getting NoFollow links. In fact, you'll want to get an equal amount of them as well. While they don't pass on link juice, they do help associate your site with anchor text (the keyword phrase that makes up the URL pointing to your site).
How Search Engines Use NoFollow
- Google follows NoFollow links but simply does not pass on credit to an outbound link that is tagged with the attribute.
- Yahoo follows NoFollow links and excludes the link from all ranking calculations.
- MSN may or may not follow a NoFollow link, but it does exclude it from ranking calculations.
- Ask.com does not adhere to NoFollow.
Finding DoFollow Sites

Don't Be A Spammer - Be A Value Provider!
Spamming Dofollow resources will not give you results...not only that, it hurts the entire community, and it persuades Dofollow providers to convert their sites to Nofollow. Be sure that all of your link-building exploits are done so tastefully and honestly. If you don't cram keywords and give an honest account of what your site is about, search engines will reward you for it!
In getting links, regardless of if they're DoFollow or not, please note that Google and other search engines are rather smart these days. If you have multiple links pointing to a page from one DoFollow source, or if that source is also linking to other places that are giving you backlinks (such as your Blogspot or Wordpress account), search engines will notice and may devalue your links to the point where they become worthless. Build links naturally and over time. Provide value and they'll be quality links. Otherwise, you'll hurt your website's link building efforts in the long-run.
By the way -- when it comes to link building, only the first link pointing to a site counts. Posting more than one hyperlink on the same page to the same source won't help in any way.
How to Spot DoFollow Links
...and NoFollow ones, too

Everything not highlighted is DoFollow. It's a quick and easy way to do a background check on your backlinking resources to see if they've retained DoFollow status!