Wordpress is perhaps the main web application that most commercial and personal blog sites are using today. It is a feature rich application specifically designed for blogging and it is also one of the most successful at acheiving high site rankings on the search engines. It is because of it's success at acheiving such high search engine rankings that most developers and SEO specialist choose to use Wordpress for their blogs.
Like most of the content management systems on the market today Wordpress's functionality can also be expanded through the use of add on modules. Despite the add-ons that are available for Wordpress, it still has it's limitations and as a matter of opinion should still just be used for it's primary design purpose as a blogging platform rather than other purposes such as social networking or basic websites.