The websites4all site makes a great randomizer. Just scroll part way down and click somewhere in the "dark" ("light"). You come out with a high chance of seeing something you wouldn't have found before. You might even find something you like a lot.
Another "randomizer" is the Recently Updated list on Weblogs.com. You never know what you'll find there.
What's not random in the weblog world? The HotList/Top 100 and Ranking By Page-Reads lists on UserLand. That doesn't seem to move much. It's kind of like expecting a new artist to outsell the all-time hits of Elvis, The Beatles, and Michael Jackson. A world record is exactly that. So let's move on. Thankfully, they do give you two time frames to use: Today and Since a Long Time Ago (currently 10/27/2001).
Here's a good write-up on web services ("Five Myths About Web Services") on John Patrick's site that I found by "clicking in the dark" (through tweney).