I've heard that SharePoint Solution Installer is a good one, but it only works for 2007 and the 2010 version only appears to be in the planning phase.
Does anyone know of any existing tools out there that work for 2010?
I agree with Marc - it should be done with PowerShell.
You might think it's not user friendly but if you write the script so it's a single command it can't be any easier for the admins to install it. The script way also allows the IT-pros to inspect what you're actually trying to do which MSI's or EXE's don't.
I do recommend you to take a look at Gary "PowerShell" Lapointes solution deployment script: http://blog.falchionconsulting.com/index.php/2011/04/deploying-sharepoint-2010-solution-package-using-powershell-revisited