I've had Trendalicious on my Firefox toolbar for a while now, and I check it every hour or so to see what's popular at del.icio.us. It's a great way to get a quick rundown of what's getting the most buzz - I've found a lot of useful links this way.

Related: Use Digg to find what's popular and useful on the Web, view Flickr's most popular tags, check out the Top Ten Viral Videos, or read about how to find the most Most Popular Searches on the Web.