nofollow is a non-standard HTML attribute value used to instruct search engines that a hyperlink should not influence the link target’s ranking in the search engine’s index. It is intended to reduce the effectiveness of certain types of spam indexing, thereby improving the quality of search engine results and preventing spam indexing from occurring in the first place.

However, this technique has been implemented as a result of spammers, and, consequently, almost all modern blog platforms adopted the tag rel="nofollow". This technique might hurt your blog commenters, who, let’s face it, sometime comment on your article just for a back link. But if they’re sincere and write something useful, why not appreciate it and not follow their links. What can you do about it? Turn off “nofollow”. Show your readers that you really appreciate them.

While all engines that support the attribute exclude links that use the attribute from their ranking calculation, the details about the exact interpretation of the attribute vary from search engine to search engine.

• Google takes “nofollow” literally and does not “follow” the link at all. That is supposedly their official statement, but experiments conducted by SEOs show conflicting results. They show instead that Google does follow the link, but not index the linked-to page, unless it was in Google’s index already for other reasons (such as other, non-nofollow links that point to the page). Links with NOFOLLOW are included in the back-links reporting data at Google’s Webmaster Central.
• Yahoo! “follows it”, but excludes it from their ranking calculation.
• MSN Search respects “nofollow” as regards not counting the link in their ranking, but it is not proven whether or not MSN follows the link.
• Ask.com does not use the attribute for anything.

So, go ahead and install the doFollow plugin for WordPress and place a link somewhere on your blog indicating that you DO follow comments. Here are some graphics options from Feverish Thoughts:

or these taken from Randa Clay:

This is what Randa Clay thinks about nofollow, and here is a great list of bloggers who support the “No No Follow” movement. And I wish this page had an English version. And here is the ultimate list of dofollow plugins.