I use cloudkick.com, which is a hosted service. It's the most accurate hosted monitoring I've ever used. Less false positives than any other hosted service I've used, and the fastest (accurate) notifications of true issues.
The catch?
You have to be using one of the hosts they cover (currently EC2, EC2 Europe, Rackspace, Slicehost.). They will probably add a bunch more soon, as they are organizers of the libcloud project, which aims to build interfaces for all popular providers. Code checked in so far covers Linode, vps.net, vCloud Express, for example. (http://libcloud.org/)
It's easy to install because you don't have to install any agents on your system. You just plug in your (provider) key at the friendly, easy Cloudkick dashboard, et voila. All the accuracy of agented monitoring, none of the mess/packages/server set-up.
(Lots of hosted monitoring services act like "agentless" monitoring, which means not that accurate or fast to notice things.)
Also, it's free. (Though it seems like they may have premium services one day.)
The catch?
You have to be using one of the hosts they cover (currently EC2, EC2 Europe, Rackspace, Slicehost.). They will probably add a bunch more soon, as they are organizers of the libcloud project, which aims to build interfaces for all popular providers. Code checked in so far covers Linode, vps.net, vCloud Express, for example. (http://libcloud.org/)
It's easy to install because you don't have to install any agents on your system. You just plug in your (provider) key at the friendly, easy Cloudkick dashboard, et voila. All the accuracy of agented monitoring, none of the mess/packages/server set-up.
(Lots of hosted monitoring services act like "agentless" monitoring, which means not that accurate or fast to notice things.)
Also, it's free. (Though it seems like they may have premium services one day.)