Wordpress navigation is the only thing in wordpress which lacks plugins. The navigation is quite hard to customize.

But I found out a very cool plugin that creates a dropdown nav bar. It is 100% seo because you can see all the links on your nav bar as "href" links in the page source code. It is very customizable through CSS Style.

Here is the link
http://pixopoint.com/multi-level-navigation/


There is another plugin called NAVT but there are two problems with that.
1. It needs java script installed on the viewers computer.
2. It is not SEO because of the javascript.

You can find this on the official wordpress plugins page. It is good for those who have personal blogs.