One of the World of Warcraft mods I picked up early on in my playing is one put out by WarcraftRealms.com - Census+. It doesn't enhance my playing at all, but it does give me a "good samaritan" feeling, and contributes to my apparent love of statistics.
All the addon does is make a series of "who" calls in the background every half hour while you're playing, and then records the population information into an LUA file which you can upload to the WarcraftRealms.com website manually or with an automatic uploader they reference on the site. From there, your LUA file data is merged with all the other submissions, and is (usually) processed within a day or two and your population information becomes part of the huge pool of population information WarcraftRealms has been collecting since near WoW's original launch date.
There's no guaranteed reward for submitting to this site, but every month the site owner buys Amazon gift certs and the likes and then assigns all submitters for the previous month "ticket numbers" from which he then draws winners from. I actually won one month, and won a $75 gift cert from Amazon, which rocks pretty hard!
As a guild recruiter, the WarcraftRealms site was pretty helpful in helping avoid recruitment of "guild hoppers", because as long as there's regular Censuses being taken on your realm, there's a pretty decent history of any one player in that realm in terms of how long they've been playing that toon, what guilds they've been in and how quickly they've progressed through the levels thus far.
As a regular player, the stats aren't as "helpful" but they're still fun to pour over on occasion. If nothing else, I like to see my personal submission stats for the site itself improving... seeing my name climb higher on the top contributers lists.
I made a list today of Horde and Alliance realms that have been up for more than 6 months but still have very limited representation in the Census submissions. If you're on one of these realms, consider picking up the addon and contributing!
Horde side:
Anetheron
Blackrock
Chromaggus
Crushridge
Eredar
Gul'dan
Icecrown
Jaedenar
Kel'Thuzad
Korgath
Lothar
Magtheridon
Mannoroth
Ner'zhul
Onyxia
Shadowmoon
Spirestone
Stonemaul
Stormreaver
Tichondrius
Zuluhed
Alliance side
Andorhal
Anetheron
Blackrock
Chromaggus
Crushridge
Darkspear
Eredar
Gul'dan
Icecrown
Jaedenar
Kel'Thuzad
Korgath
Magtheridon
Shadowmoon
Spirestone
Stonemaul
Warsong
Zuluhed