[kels]

I have no affiliation with any of these companies.

I use StatusCake for basic uptime monitoring for websites. I switched to them from Pingdom because they are cheaper. Only downsides I've had with StatusCake is if something is down it doesn't give you the cause. Pingdom would show you the trace route. That has made it hard to tell and I would get quite a few false positives saying sites were down. I haven't had false positive issues for months now though. Their paid plans have SSL/domain monitoring.

Monitis for cheap Linux CPU/RAM/Load monitoring.

[abrongersma]

I've also had great success with StatusCake. The SSL cert monitoring is a lifesaver.