Monitis.com - basic is free. can also install an agent on server/PC to report to cloud for emails and SMS alerts.
Another one I have done some looking into to augment SolarWinds is AppNetta http://www.appneta.com/
I was oringinaly looking to them for end to end monitoring with remote sensors but now that - that will be possible with Solarwinds upcoming not so much. However, the external cloud to our DMZ internal is still important so we are still looking into them.
However, when things go down on the internal adding an mobile relay device to send out alerts during the outage is very useful. I set up a private twitter feed and the have the alerts sent to that feed that only the team(s) can subscribe to for these alerts during outages.
It's probably worth mentioning that SolarWinds just acquired Pingdom.com, and that (monitoring from external locations) is their sweet spot.
Just in case you want to keep it in the family.