If you are under the impression that there is only rel=”nofollow” tag, you are about to learn many more of them.
I have come across these tags which I never knew existed. I was familiar with only couple of them like rel=”nofollow” and rel=”me”. I use ‘nofollow’ attribute quite regularly, for every single outgoing link. But with these tags I can indicate the search engines exactly what kind of page I am Linking to.

Rel=”Nofollow”

Almost all webmasters should be familiar with this attribute. It lets you you tell the search engines not to pass Link juice to the linked article/ Post or Website.
Example: <a href="http://www.geekywood.com/" rel="nofollow">Tech Gadgets</a>.

Rel=”me”

If you link to your profiles at any social network from your website/ blog, then instead of using rel=”nofollow” to the links,
use rel=”me” attribute. It tell the Search engines that the linked page belongs to me and is my profile.
Example: <a href="http://geekywood.stumbleupon.com/" rel="me">MY SU profile</a>

Rel=”Tag”

I have seen so many Wordpress sites using plugins that automatically link related text used in a blog post to a Tag page and others link manually. If you do that, you should stop the practice because Google hates duplicate content. And when you link to ‘Tag’ page, you refer Google or any other search engine to Duplicated content. Instead you should use Rel=”tag” tag to the linked text. This tells Search Engines that the anchor text used is a tag used to easily find the page and should be used as a keyword.
Example: <a href="http://www.geekywood.com/tags/SEO" rel="tag">SEO</a>

Rel=”Bookmark”

Bookmark is a link to a key entry point within an extended document. Using this tag to link to your Permalinks in blog posts
helps Search engines identify the main page or original content page.
Example:<a href="http://www.geekywood.com/2007/11/web-hosting-and-digg-effect.html" rel="Bookmark">Digg proof Hosting</a>

Rel=”License”

This tag can be used to link to licenses of the current page or article. When you use rel=”license” attribute to the outgoing link, it indicates to the Search Engines that the destination of that link is the License of current page.
Example: <a href="http://creativecommons.org/licenses/by/2.0/" rel="license">Some Rights Reserved</a>

Rel=”directory”

By adding this attribute to the hyperlink, you indicate to the search engines that the destination of the link is a directory listing containing an entry for the current page. It can be used when some Directories want a reciprocal link from your site to get featured in the Listing.
Example:<a href="http://www.blogcatalog.com/directory/computers/hardware/modding" rel="directory">Modding Directory</a>

Rel=”Payment”

This attribute is used in the links when you are linking to a site that accepts payment. If you accept donation on your site via PayPal, you should use this attribute in the outgoing link.
Example:<a href="http://www.amazon.com" rel="payment">Buy this e-book</a>

Hope this article helps you. A point worth noting here is that all the above tags work with Google but others like MSN or Ask.com may or may not consider the attributes like the Google does.