Confluence is available for a free license for non-profits who can’t afford to pay. Our local java users group (SACJUG) fit the bill for this, though no one’s ever been quite organized enough to officially register as a non-profit.
At the last SACJUG meeting we debated over what wiki/content management system to use and eventually settled on Confluence. The feeling was as a java users group we needed to use a Java based tool. I volunteered to see if they’d grant a license since my company is already a Confluence customer. About a week later I got a nice email from Atlassian granting SACJUG the right to use Confluence for site.
Kudos to Atlassian for the program for non-profits. And if you’ve tried several wikis and they never caught on you might try out Confluence. It’s really taken off in our organization and the pricing is really reasonable.