- Google Sites - free wiki from Google. Perfect choice if you need to save some money and can keep your data in the cloud (people in corporations not always can).
- Confluence - great product from the great company named Atlassian. We use it at the Apache Camel. Can can host private instance of Confluence starting from 10$ per year (or pay Atlassian 10$ per month to use their cloud).
I've also worked with Alfresco and JSP Wiki, however I don't recommend them. Too complicated and unintuitive comparing to the Google Sites and Confluence.
I'm curious about your experiences with wiki tools. Any feedback will be highly appreciated. :)