Sorry if this is a bit off topic, but I'm building a website to promote the video's I'm developing with Kdenlive, and want the website to include forums like these. What software or web development package was used to create the forums on this website? Is it freeware? Thanks.

Drupal I believe. Also take a look at Wordpress.org, you could consider looking at a web host that allows you to install these as part of a hosting package rather than going for a free site from wordpress.org for example and then having to pay for each additional module.

But be prepared for a lot of work if you host a forum with regard to deleting spam, there are numerous users here deleting spam daily.

For paid hosting Squarespace is another option but there are thousands of hosting companies out there to look at, I'd forget any idea of hosting it yourself on a domestic net connection though if that was your intention.

vBulletin is probably most widely used forum software.