This has be the best offline blog writing tool I have seen to date, and it is free. Live Writer lets you create and edit your weblog entries just like a word document, without the need to be constantly tweaking the html. It is compatible with DasBlog, Blogger, LiveJournal, TypePad, WordPress, and of course Windows Live Space as well as others.
One of the problems I had with some of the other tools was a quick way to link my images to thumbnails. Live Writer has a very cool feature to help you do this. You simply "Insert Picture" and edit the properties to link to the original image at full size.
The WYSIWYG support is as good as anyone could want. You can switch between Normal, Web Layout, Web Preview, and of course HTML. It is smart enough to collect all relevant information such as headings, fonts, colors, background images, margins and block quotes and show you a true view of what to expect.
Another excellent feature is it's rich plugin support. It is still in beta, but already has a vast array of available plugins including Code Formatter, Inserting Maps from Windows Live, Inserting video, and pictures from Flickr.
Download Windows Live Writer.