I think I have ever posted a thread here, asking about migrating a website from Joomla to Wordpress? Anyway, I have found a good plugin for that, named Joomla/Mambo to WordPress Migrator. I have tried it and it works well on my blog. Just want to share.
It migrates posts and other textual contents, internal links (I am not sure about the external ones), and databases. You need to manually move your images and videos to the proper folders on your blog.