I have to disagree with everyone here putting MediaWiki out there as the default best wiki software. Though it's widely used, well supported and of course very well known because of Wikipedia, it is hardly the best choice for every use.
It's important to remember that MediaWiki was and is developed primarily to host Wikipedia and related projects. It's unlikely your wiki project shares the particular requirements of Wikipedia (the 7th busiest web site on the Internet, and hosting encyclopedic information).
Though I am biased towards EditMe - www.editme.com
- I would advise anyone with this question to use the WikiMatrix.org
web site - it provides a nice comparison of the many wiki products available based on the specific needs of your project.
It may very well be that Wikimedia is the best choice for your project, but based on your stated requirements (simplicity, easily customized) I'd say there are many better options. EditMe happens to excel at both of these requirements.