InterMapper (commercial) is what I use -- It's very flexible and the company behind it is pretty responsive to enhancement requests and each release of the software has brought a bunch of new (useful) features. There is an annual license agreement, but I don't find the pricing prohibitive, even (especially) for small clients.
InterMapper also has a database backend that lets you do trending/reporting (similar to what you can do with Cacti, etc.), though this isn't very well polished yet.
I'll be the contrarian and say I don't much care for Nagios: partly because I'm not a fan of the "remote plugin execution" mdoel, but mostly due to bad experiences with setups that had an abysmal signal-to-noise ratio and what I find to be a less-than-refined configuration process.
Edit to respond to Questioner's Edit: With InterMapper you'll almost certainly have to run SNMP daemons on the stuff you want to monitor, and you'll probably have to customize some of the thresholds per-server unless your environment is really tight, but it's all done from the GUI & pretty easy.
Stuff like hardware monitoring (drive failures, etc.) usually requires a custom probe, but there are a bunch of them already written (and if the probe you need doesn't exist implementing it yourself is pretty easy).