If you are like me, finding it painful with the internal blog editor in WordPress, I highly recommend to use Windows Live Writer, a free blog editor software which also supports Blogger, Windows Live Spaces and other popular blog platforms. Windows Live Writer is included in the Windows Live package together with Live Messenger and other modules.

Screenshot:

A glimpse of features:

1. Allow you to blog on web content directly with "Blog this" function in Internet Explorer and Firefox.
2. Publish video, images and media at ease.
3. Link glossary to gather your commonly used links.
4. Add post slugs and tags (keywords) directly while writing your post. Support external tagging like Technorati.
5. Ability to resize images, by dragging corners directly or picking a predefined size.
6. A range of additional plug-ins to add more capabilities.
7. Automatic links creation with URLs via Smart Corrections.

Check out the complete list from the official site.

A few useful shortcuts to speed up your post writing process:

Ctrl+K - Insert Hyperlinks 
Ctrl+L - Insert Images from the desktop or the Internet
Ctrl+S - Save drafts locally
F7 - Run Spell Checker
F9 - View or Hide the Sidebar

More shortcuts can be found here.

There are also some handy blog editors such as Qumana and ScribeFire, which is a firefox extension letting you blog while surfing. But I always stick with the windows live writer in writing entries. The software is light weight and features-rich. It turns blogging into a pleasant experience. This is in my view the best free blog editor software.