A wiki is usable for number of applications. There some 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