The United Nations Educational Scientific and Cultural Organisation recommends and lists Tikiwiki as a Productivity Web Tool:
Tiki CMS/Groupware (aka TikiWiki) is a powerful open-source Content Management System (CMS) and Groupware that can be used to create all sorts of Web applications, Sites, Portals, Intranets and Extranets. TikiWiki also works great as a Web-based collaboration tool. TikiWiki is a multi-purpose package with a lot of native options and sections that you can enable/disable as you need them.
It is designed to be international, clean and extensible. TikiWiki incorporates all the features present in several excellent wiki systems available today plus a lot of new features and options, allowing your wiki application to be whatever you want it to be--from a simple wiki to a complex site for a whole user community with many intermediate steps. You can use TikiWiki as a forums site, a chatroom, for poll taking, and much more! The possibilities are endless. You can see some Tiki sites in action. Major features include articles, forums, newsletters, blogs, a file/image gallery, a Wiki, drawing applet, trackers, a directory, polls/surveys and quizzes, a FAQ, chat, a banner management system, Webmail, a calendar, Ephemerides, maps, charts, Mobile Tiki (PDA and WAP access), RSS feeds, a category system, a theme control center, workflow, live support, Shoutbox, ACLs, and more.