I set up a WordPress blog in about 15 minutes. It's easy to use, well documented, well maintained and under constant development. It has been around for a very long time and there are hundreds if not thousands of extensions and themes available for it. You can set it up as a single blog (
WordPress) or if you like you can use
WordPress Mu which will allow you to host multiple blogs in a single data file.
You can also set up a blog at
www.wordpress.com and link it to your site.