Computer and Video Games has put up an article looking into some of the custom Portal maps that have popped up online. The site takes a look at some of the best, such as Ren_Test2, along with some not so great maps.
Created by one of Bethesda's Fallout 3 team, the internet won't shut up about Ren_Test 2, one of the most mind-numbingly taxing Portal maps ever created. So, take it as read that this map is so bloody brutally challenging that it makes Portal's advanced levels seem effete.
They're pretty honest in their reviews (as far as I can tell, I need to try some of these out myself), so go check it out and give the maps a try for yourself.