In ancient times, before WWW, folks dialed in to bulletin board systems for sharing. Citadel was there. Lately Citadel has been updated to provide many more means of access. The most popular is likely web access using a browser.
Citadel is a suite of packages in Debian GNU/Linux which you can install in a minute or two. You can install it on a PC or a server to provide web services, LAN service or just to serve your local PC.
Unlike phpBB and other PHP scripts, Citadel is written in C so is a bit faster than an interpreted web application. This allows it to scale better and serve a growing organization. It even has clustering so if you ever do max out a server, you can add additional instances and keep going.
I became interested in Citadel when I read that Largo FL was not happy with Evolution + Groupwise from Novell. I was curious what alternatives there might be. Citadel is trivial to install and works very well for many purposes. Compared to the $25K per annum that Largo spends on their system, Citadel has very high performance/$. Try it out. I think it can be useful for an individual, family, or small to medium sized businesses or organizations. Check out www.citadel.org
I created a short video showing the installation and initial usage of Citadel in a virtual machine: