Ask HN: What tools do you use to monitor your servers?

I recently ran into a problem where MySQL was running out of connections. For now I've upped the maximum_connections setting and I'm keeping a close eye on the server.

I'm sure there are lots of tools that could monitor this sort of thing for me, and alert me if any problems occur (or ideally before they occur).

I suspect there are also tools that would make it easier for me to diagnose the problem, and allow me to see what was happening on the server at the time.

So my question for HN is: What tools do you use to monitor your servers?