For checking up on sites in the enterprise, I use Alertsite. It was suggested to me by a VP I work with at McCann, Ed Recinto. It’s been a great tool.
For personal websites that I manage, I’ve been using something I rolled in newLISP, sitebeagle. Why? Because beagles are great watchdogs.
Very often, most problems can be solved with tweaking code, changing permissions, or upgrading and apache or mysql.
Very often, it’s the weekend, I’m sitting in a cafe, and get an alert from Nagios or Alertsite. With iSSH, on the iPhone, I can ssh into a LAMP server and do the work I need.
I can see things getting a bit more complex. What tools do you use to sysadmin from an iPhone?