1. Slashdot. For the interplay of how information feeds into the interface design, how the whole system is tweaked to serve very high traffic efficiently, and the interplay of those two challenges.
2. Classmates.com, for the brilliant-yet-evil way they suck you in, offering you a taste, getting a little info about you, giving you a little more, getting a little money from you, and so on. The lure is very strong; I can understand how they afford all those pervasive banner ads. For students of permission marketing this is a must experience.