A Wiki is very easy to set up. However, before setting one up, you should consider if a Wiki is the right approach. Blogs have gained significant ground over wikis in the past few years. Forums are also good for certain applications.
For Wiki software, I recommend MediaWiki (Free):
Download - MediaWiki
For blog software, I recommend WordPress (Free):
WordPress › Download
For forum software, I recommend vBulletin (Good value for the money):
vBulletin - Downloads