This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
I once installed JSPWiki and found it easy to get started with. It would probably be my first choice if I was looking for a Java-based wiki, since it seems to be the one mentioned most often.
For a high-end solution that requires heavy-duty customization capabilities (meaning Java code, not just templates) I might go for XWiki, but that has a much steeper learning curve (while also offering more features).