Inspiration for blog posts can strike at any time. The fact that blogs are by definition on the web, this can make it difficult to post if you aren't online. In the past, I've written a few posts in a text editor and then copied them over to WordPress later, but that process is a pain.
After doing a little looking for an offline blog editor, I saw several references to folks using Windows Live Writer and being quite happy with it. Originally designed to work with "Windows Live", which I don't use, the Writer component works with other blogging platforms as well. This includes WordPress, which is what I use on my site.
So, I'm trying it out now (literally making this post in it). At first blush, it seems pretty nice. Aside from being able to write posts when not directly connected to the blog, I also like not having to work inside the browser itself. While the editor on the WordPress page is pretty nice and Firefox adds spell checking in the form, it's still not a very elegant experience to work inside the browser.
One of my biggest annoyances is that you have to scroll down in the browser to see the categories. The desktop app nature of the Writer application allows those categories to always be visible in selection menu at the bottom of the application. I often forget to add categories because they aren't in view. Not an issue with the desktop Writer. Also, there is an option in it's preference to trigger a reminder to add categories before you post if you didn't include any. Very nice.
Now, most of these User Interface issues could be solved in the browser, but overall, it's MUCH easier to tackle that stuff inside a desktop application.
I did have one issue when trying to install the software. I keep Firefox set as my default browser. After trying to unsuccessfully to install the Windows Live Writer a few times I temporarily set I.E. to be the default browser. After that, the install worked as expected. So, if you want to try it out, keep that in mind.
There are several other tools built into Writer that I haven't tried out yet, but the top level it seems to do what I want quickly and easily and basically get out of my way and let me create posts. That's the sign of good design. When it does what you want it to do without really having to think about it.