I believe that MarsEdit is the premier weblog editing software for Macs. 
Before beginning to use this program over a year ago, I did a good bit of research into all the available options and concluded that it best fit my needs.
One of the main things that I like about MarsEdit is that it resides on your harddrive, not online. This gives you the ability to draft and edit posts at your convenience, whether you are online or not. The other key features that appealed to me (then and now) are:
- Ease of use — both novice and advanced users can use it without a steep learning curve.
- Importing — easy importing of information from web browers and/or newsreaders to use in your posts.
- Uploading — files and images can be easily uploaded to your blog.
- Preview — ability to see in real time what your post will look like on your blog.
- Other — assign one or mulitple categories and sends pings with each new post.
MarsEdit runs on Intel and PowerPC Macs, and a single license costs $24.95. The developer offers a fully functioning 30 day demo, and I urge you to check it out if you’re interested in blogging software.