Lauren Wood, of The Gilbane Report has an excellent introductory article on the corporate use of Blogs and Wikis (factoid: did you know that "Wiki" is a Hawaiian word for "hurry" or "quick"?). The article gives real world examples of companies using blogs and Wiki's for internal and external communication purposes.
I agree with Lauren that these tools have great potential in the enterprise. Blogs and Wiki's support information sharing needs that get miss-handled by email. My metaphor for email is a big heavy crescent wrench that gets used for everything from whacking something like a hammer to tightening and loosening the smallest of nuts -in the process, destroying both the wrench and the things you are trying to fix with it. Email is (miss)used as a file system, database, audit trail, and discussion tool. As a result, almost every one I know complains about how unmanageable their email box is.
Wiki's and blogs are a happy medium between totally unstructured email and extremely structured and formal content management solutions. Like email, these tools give users a low hurdle (technical, organizational, and process) to contribute valuable information. However, unlike email, information within these systems is less vulnerable to being lost and is more accessible to a wider audience - especially when coupled with a good search engine.
The article also discusses the risks of using Wiki's and links to a very good blog posting by Leigh Dodds about how to make your corporate Wiki succeed.