Your Annual Whois Update Notification
Per ICANN's requirements, ICANN accredited registrars have to make sure the whois information supplied by their users is correct. One of the ways they have to make sure this is the case, is by annually sending them an email and asking them if the whois info for their domains is correct.
It should be obvious that this measure doesn't really stop people from putting fake information in the whois record of their domains. But ICANN require sit so, most registrars comply.
The technical implementation of this requirement differs from registrar to registrar. Some, like Webair for example, do it in a way that makes sense and minimizes the potential hassle for their clients. Others, like Moniker, prefer to annoy their clients (so it seems). Every year Moniker sends its clients an email with the subject "Action Required: Your Annual Whois Update Notification". Inside is a link you have to click to verify your domain info is correct. Doesn't sound to bad? It wouldn't be if it were only 1 link. In reality it's 1 link per domain. If you have 500 domains at Moniker, you'll have to click 500 different links. In a modern browser that also means you'll have 500 tabs to close afterwords. Why a registrar (that prides itself in offering solutions for those with large domain portfolios) would require someone with 500 domains to jump through 1000 hoops just to keep their domains alive, is beyond me.