I been using alerta but i just got a bill for 100 something dollars bleh, then searched the forums and came across a thread with http://mon.itor.us/ in it which seems pretty awesome but it would be cool to have something like http://mon.itor.us/ but I wanted to see what everyone else uses.
I've played with this http://www.serverdensity.com/ and it seems pretty cool. Little more expensive than mon.itor.us though.
http://www.hyperspin.com
The free version is good, it can get pricey if you have an number of servers
They have servers all over the world to monitor your site so that's pretty cool.
Even the free version was able to point out a bad nameserver/DNS my hosting company was using.
I never used this one but I've been using monitis for 1 year and it's awesome.
We roll nagios internally. Theres some pretty good monitoring software out there as stand alone software, services I've found to be rather lacking.. its an idea we've thrown around for a bit now, maybe we could ask you guys to help us add features to it.