The company I work for has a site-wide license. Before Confluence, we used Lotus Notes and other proprietary databases (we joked they were data roach traps - data entered in, very/very hard to get out). After a small pilot, Confluence spread quickly through our company and is now indispensable. The only thing that could replace it is probably Sharepoint. Confluence is basically the concept/power of the Wiki without the Wiki-awkwardness. A normal-user friendly Wiki.

Confluence has traditionally free for open source projects. I upvoted this because I think this limited-time offer ($5 for 1 year) would fit many small, distributed startup's needs for a centralized knowledgebase (if they are not already using Drupal or Backpack). Jira is also included in this $5 offer too [separately - not 2 for 1] (we use that too - our testers/business analysts love it)