I use Windows Live Writer as my principal blogging tool. It has a neat feature which detects your blog's look and feel - it's style. LW then emulates that style as you are using LW to create or edit a blog post. This is a neat feature that I personally like a lot.
LW achieves this by creating a simple testing post on your blog, then deleting it. The only problem with this is that some blog posting tools pick up that test post immediately, as described in a post by Josh. When Josh posted his entry in the summer, there were a mere 860 its - but today Technorati lists 9592!
At first sight, this may appear to be a low number of users. But in order to have been picked up, either your blog is be hosted on an "when it's updated, we'll ping someone..." type site or you have configured LW to ping some blogging service. So I suspect that it vastly undercounts the number of LW users, but it's certainly fun to watch. From the look of the stats as I was creating this post, the hits keep rolling in!