Windows Live Writer (WLW) by Microsoft is an application for composing and managing blog posts. You can write your posts offline and publish them later. There are many helpful tools in WLW that make creating popular posts easier:

WYSIWIG using the theme of your blog
Image manipulation
Spell check
Easy HTML tables
Include maps from Virtual Earth
Automatically syncs changes before re-editing posts

See instructions here:

http://en.support.wordpress.com/xml-rpc/windows-live-writer/