Let us list some: (Let us consider some structured elements sucha s from, to etc.)
Email - Mainly unstructred. But some people may adopt standards such as using specific tags in subject line - to put some structure in information. This structure is typically used for classification etc. But it has good handle on people involved and so on.
Discussion boards and Newsgroups. Good classification of topics, but unfortunately nothing more than that. Often, the user IDs are not unversal.
TWiki. We will come to this later.
More to explore: How to build systems that will allow us to manage both structured and unstructured information in single framework?