RSoP can be easily accessed from within Active Directory to see what policies apply to a workstation/user. To immediately test a GPO, first run GPUPDATE on the domain controller, then run GPUPDATE on the workstation.
Software distribution via GPO:
You can install MSI packages via GPO by "assigning" them to computers (full install) or users (installed at first use). Assigned packages are installed during Windows startup, before the login screen is shown. You can also make MSI packages available in the Control Panel via GPO by "publishing" them to users. For a walk-through on how to do this, see Brian Posey's article. To create your own MSI, check out this list of installers. I should try out the free Advanced Installer.
To learn more...
http://www.gpoguy.com/ - news, free tools and training videos
http://www.gpanswers.com/ - message board w/ an emphasis on advanced concepts