That just lists the applications available in the Maemo.org (extras) repository. This repository should be configured on your N900 by default, so all those apps will be available via App Manager (as stable versions - the unstable versions will only be shown if you've specifically configured the extras-testing and/or extras-devel repositories). App Manager (or Ovi) is the preferred way to install applications, as it'll automatically handle any dependencies that the application has.