slashdot -- it used to be really useful, interesting, etc. Now HN is much better.
Yeah, I do like Slashdot's time-linear format better than HN's fluid ranking, in that you can catch up from where you left off. However, the discussion in /. has gotten really old and bitter. HN has its own pervasive negative community aspects from the other direction, but certainly has more meaningful content in the discussions overall.
The two sites still share a large percentage of links, though, and I read stories on /. that I didn't happen to catch on HN's ephemeral front page.