wikis yes. forums.. not really unless you abuse the bug tracker. Github+Google Groups seems to be the complete package these days. BitBucket also provides website/wiki/issue tracker/source control features.
Yeah, it's probably a good solution for new projects. The problem with existing projects is when you already have a quarter million posts in your forum and don't want to lose them. Although maybe a mix of providers - keeping forums on SF and moving the rest of the project might be a solution.