Allura is an open source implementation of a software "forge", a web site that manages source code repositories, bug reports, discussions, mailing lists, wiki pages, blogs and more for any number of individual projects. SourceForge.net is running an instance of Allura. It is extensible in several ways, most importantly via the notion of "tools", but also with themes, authentication, and various other pluggable APIs.
Allura is an effort for the community: an open source platform for sharing development. We think it should be of the community as well. We want to encourage community involvement in development, testing and design. We do that with a public git repo, a bug tracker, a discussion list and an IRC channel. The source code for Allura is available under the Apache License 2.0. Allura is currently managed by SourceForge, but it's in the Apache Incubator and will eventually transition to being managed and hosted by the Apache Software Foundation.