Press This, using the regular Add New Post screen
Press This is neat, but wouldn't it be a lot neater if you had access to all the functionality you have on the normal post editing screen?
With this plugin, you will have access to all the meta boxes, not just Categories and Tags.
Also, plain URLs are inserted in the post content, letting the Embeds feature do the rest.
Important: After activating the plugin, you will have to re-add the bookmarklet, from WP Admin -> Tools.