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Semiologic
Denis has a plethora of plugins for WordPress, and his CMS oriented theme utilizes them in an organized fashion to allow for the easy adaption of WP as something more than a blogging engine. Sure, it can be done without Semiological, but if you are looking for a one stop, top-notch solution, look no further.
Il Filosofo
In the ever expanding uses of WordPress, this plugin offers more options in using your install. Allows for the WP install to be in root, then set a Page as the default home page, then posts, or how ever you use the posting feature, to be in a subdirectory. Will be giving this one a twirl, as I’m working on a WP install as a CMS now. Will give some feedback.
**EDIT– This plugin is not recommended. It is not something that can easily be deactivated, and unless you are familiar with .htacess, you will bork your blog.