Region Info is an astoundingly valuable new web-based service from Metaverse Business, the Second Life user data research firm of Louis Platini (his avatar name), who does great data gathering on SL groups, SL land prices, and much more. As the name suggests, Region Info gives you a snapshot of average
unique visitors on a given Second Life sim during a given day, week, or month. Above for example are recent daily visits to Waterhead, where I keep my office, which is also the site of an official Welcome Area -- at any given moment, the average is 20-25 avatars, quite busy. (Many regions I've fed into Region Statistics average a handful or less.) Basically, this is like comScore, the web-tracking service, but for virtual land instead of websites.
Louis gathers this data from Metaverse Business bots which regularly and silently move through the world, counting avatars. And to answer the next inevitable question: Yes, Metaverse Business' bots also count other bots, and include them in Region Info stats, so this is not a perfect measurement of actual users. (Then again, what is?) However, I suspect you could use Region Info to deduce which sims are likely to have bots, since those will probably have a uniform base level of visitors.