Ask HN: What do you use for monitoring your servers?

I am working on a side-project with a few friends and we are almost ready to launch. One thing remaining (and a gaping hole!) is that we don't have any sort of monitoring set up for our VPS instance.

We want to monitor things like CPU load, disk usage, response times for nginx, MongoDB load statistics etc. Is there a tool that other startups use and recommend for this?

I've looked into Splunk but wanted to see what others are using.