Developer: Automattic, Inc.
Price: free Download on the App Store
I am writing this post on the wordpress app for iPhone. The app is free, and extremely self explanatory. It's very easy to write a new post, edit a post, or even take and attach a picture to a post. You can select tags and categories just as in the web version of the service
If you don't know wordpress, and are using another blogging system such as blogger or typepad, you should check out the latest version (2.6) of the wordpress publishing platform. It's easily the most advanced out there