Blog This - This is like the Tumble bookmarklet for quick blogging - select some text on a web page in IE or Firefox and click the Blog This button - it will launch a new blog post prepopulated with the title, text snippet and URL of the current webpage. [example]
Insert Video - Use this to quickly embed video clips from YouTube, Google Video or Myspace Videos into your blog posts. Just type the URL of the video and the plugin will generate the code for you.
Flickr4Writer - If you are a Flickr member, this plugin will help you browse your Flickr photos and insert them into your blog posts.
Social Bookmarks - Add social bookmarking icons (like digg, del.icio.us, furl, yahoo!) in your blog posts that will allow readers to save or share your articles on these social sites.
Screen Captures - This is like an offline version of Picnik Screen Capture. Specify a webpage URL and this plugin will insert a screenshot of that page in your blog post that you can later resize or apply image effects.
Syntax Highlighter - Recommended for geeks and developers who frequently share code on their blogs. This plugin will highlight the code syntax and language specific keywords in various colors.
WebSnapr - Allows you to insert thumbnail images of web pages in your blog posts similar to ones that you see on Alexa or Snap.com
Insert Code - It formats a snippet of text in a number of programming languages such as C#, HTML or JavaScript. Makes the source code more readable.
For additional Windows Live Writer plugins, please visit the Live Gallery and the excellent WLWPlugins.com. Want to write your own Windows Writer Plugin ? Try this useful guide by Keyyan Nayyeri (uses C#).