There are several blog services to choose from. Here are a few recommendations.

One of the industry leaders is WordPress.
You can get a free blog from them or install your own on a website.
Wordpress Blog, check them out for more information.

If you have a Google account you have access to their blog: Blogger
If you do not have an account you can still sign up for a free one.
If you do, you can access it from your Google Business Service account. 

Joomla CMS 1.5 can also be set up as a blog, you can use many of its extensions along with creating the look and function of a blog website.

Some of the others are TypePad and LiveJournal.

Some of the services have their own blogging tools like MerchantCircle, Joomla also has extensions for internal blogging.

What it comes down to is matching your needs with the service and features, most of the ones listed will do the job, it become more about features, familiarity and exposure with the service.