I've looked at a couple lists and directories about MT Plugins - a topic I'm just getting interested in - but couldn't find one that lets you rank posts by popularity, based on referrer stats.

Update: thanks a lot Phil for the detailed answer (sorry for the broken email link (removed since.)) Our URLs at MarketingFix are title-based too, so the entry ID won't help, but we're using PHP so the other solution should work for us. I wonder how well this will scale though. I guess it wouldn't be reasonable to extend the "most popular" calculation beyond the last few days, at least for sites with significant traffic. I really need to educate myself about MT.