A wiki is usable for quite a few applications, because there few requirements to use it and the content can be anything. You can use it for everything that can be displayed by linked text pages.
Examples
- personal notes (instead of wasting paper)
- conference protocols
- to-do lists
- news on wiki homepage, asynchronous communication
- common creation of documentation
- brainstorming
- personal or collective link lists
- efficient alternative to e-mail or phone
- much more