Tubepress is a free wordpress plugin. It allows you to merge youtube into wordpress.

So you want the easiest way to download YouTube videos or import them to your WordPress blog?
Why create a new post and use embed video to post a simple youtube video? Tubepress will make this task simpler. Create a niche videos site, search for a keyword and import all related videos from youtube and post them into your blog.
You can of course select from the admin menu what parameters you would like to import. You can choose the title, description, rating score, video length, upload date, author, number of views.
You can customize the video player size to fit into your blog. You can even choose to add google adsense into the post and get some revenue. The adsense is also customizable from within the TubePress setup menu.
You can also choose the ThumbView option which forces wordpress to use a thumbnail of the videos in multiple posts pages like (Main page, Archives, Category).