Some years ago I started to use a wiki for team communication in the projects that I was working on. At the time I had found TWiki to be the best alternative to be used in a an enterprise intranet environment. Then we discovered Confluence, and that became our new standard which started to be widely used by other teams in the company. And it has become very popular with a lot of open source projects, since it is free for them to use it. It is a wiki on steroids, and I really liked it.
But now at Solidware we were looking for an alternative to Confluence and after some research, I found MediaWiki. This is the wiki that is behind the famous Wikipedia and looks like a really nice wiki to work with. Installation was very simple and it only requires Apache, PHP and MySQL. So now we are starting to use it and I really like it. It has nice features like discussion and watchlists, the interface is really clean and it is easy to customize.
So if you are looking for a nice wiki, a collaboration application or both, I highly recommend you try MediaWiki. You can also check other sites that use it and get ideas for your own installation.