JetSpeed is your best bet. It's free, it's from apache, and there are good tutorials. My signature links take you to some free tutorials on getting JetSpeed running, although my site has been undergoing 'maintenance' for the last little while. Should be up and at them soon.
JetSpeed is great for learning, and from an enterprise standpoint, is only as good as the hardware it runs on. Obviously, if you are creating an enterprise portal for a client,
alot more should go into your portal decision other than "easy to use." But for a good place to start, I think JetSpeed is your best bet.
-Cameron McKenzie