I've often been unhappy with off-the-shelf monitoring systems, because every major site I've worked on has had some unique kind of problem.

For instance, there was one site where user abuse was the real problem... Bad enough that I built something that detected possible abusive behavior and would beep my pager.

For another project we had about 20 geographically distributed mirror sites, and we had to monitor network connections to all of them and make sure they were all alive and staying synchronized.

Right now I've got a site where the caching system screws up periodically and then I start getting 500 errors. Sooner or later I'm going to really fix the problem, in the short term what I really need is something that gets in my face whenever the 500 error rate spikes.