Hey, it's that new guy!
That's me! 
I beat portal a few times, it kinda lost it's replayability, so when I got it from steam, was I ever happy. I went through and got the camera shy achievement, and I started playing some custom maps.
I can't ABH, but who cares? I have my mouse sensitivity at around 13.5 right now, working it up to higher, and I'm getting better at this whole portal game.
I never listened to the dev. commentary, so I'm running through the game again so I can.
If anyone wants to suggest some great custom maps, or some great maps that show advanced techniques and help you get better with them, that'd be great!
I'm looking forward to making some maps of my own, but not anytime very soon. 
I'll get an avatar up eventually, as well. I promise.
Well, try every map on this site in the Releases subforum. On this site, quality is guaranteed. On other sites, most maps are very, very sh1tty, to say at least. And those of the quality ones there are here as well 
(percentage calculation sphere self-test failed!)
108.1% done, -8.1% remaining...
Released here!
All maps for Portal (1/2) worth playing are mirrored here.
I'd try them all, the term "great" is somewhat subjective. But if you want to start off with a really good custom map, try Manic Mechanic or Boxy Trixy. Those two are at the top of my list of great maps.
Portal TFV map pack!
they're all good. except for the lame ones. manic mechanic, the watcher chambers, TFV, and PMK are all awesome, just to name a few.