Portal: Awesome game. Complete it, then try these. Skip to step 3 if you don't have Portal. Or go download it on Steam. It's worth your $10.
1: Rens test maps. Hard, single room maps. #2 is awesome. #1 is ok. #3 has some good concepts but has some other issues where I don't think it qualifies as portal. (trying a known tactic over and over and over until the physics works just right for what you're doing) #2 is well worth your time.
http://www.fileplanet.com/184974/180000/fileinfo/Portal---Ren_Test-3-Mini-Map-Pack
2: Portal Prelude, 19 level story-based maps on the events leading up to the original portal game. Fan made, supreme quality, very fun, very immersive.
http://www.portalprelude.com/download.php
3: Fun 2d web-Flash version. Good fun.
http://www.newgrounds.com/portal/view/404612
4: Proper Portal Mod created from the 2d flash version. Great implementation!
http://portalmaps.wecreatestuff.com/
5: Unlimited stuff to try at one of the primary fan sites.
http://forums.thinkingwithportals.com/
6: Awesome original song and machinima for Portal.
http://www.youtube.com/watch?v=b6Q_koTHB54&feature=player_embedded#!
