Best software to use for Forums, such as Amiga.org, any platform.

[Argo]

I'm fond of phpWebsite.
http://phpwebsite.appstate.edu

phpWebSite provides a complete web site content management system. Web-based administration allows for easy maintenance of interactive, community-driven web sites.    phpWebSite's growing number of modules allow for easy site customization without the need for unwanted or unused features. Client output from phpWebSite is valid XHTML 1.0 and meets the W3C's Web Accessibility Initiative requirements.    Founded and hosted by the Web Technology Group at Appalachian State University, phpWebSite is developed by the phpWebSite Development Team, a network of developers from around the world. phpWebSite is free, open source software and is licensed under the GNU GPL and GNU LGPL.

[Argo]

Nope, you can host it anywhere, as long as the host offer PHP 5. Which they all should by now.