ListAtlas makes Twitter lists simple by ranking them based on the number of followers. The current leader being the list for the Twitter team themselves. The top 5 is rounded out with two lists from Mashable and two lists from Scoble. Each with a minimum of 2600 followers.
You can also use a handy search on the right side to find lists based on any keywords you wish. My search for some entries didn’t find the result I know was there, so I am not sure how the search is being built just yet. If the ability to follow the list could be added right from the search result, using oAuth, that would be excellent.
It wouldn’t take much to get into the top 250 lists on the site, with the lowest having 138 followers. I am not sure how they are building and scanning every list in Twitter, but the author states he runs a script once a day to bring them all together.
Overall a great quick tool if you have to be like everyone else and find the popular lists.