You should try PolyMon, a .NET based, open-source monitoring tool on CodePlex:
http://polymon.codeplex.com/
At least for our case, it hit the sweet-spot of functionality and a lean and easy setup.
You can choose from some out-of-the-box tasks like Ping or URL monitoring, but you can also easily implement your own more complex tasks. Works quite well for us.
The tool itself is not distributed, but you can easily set up two instances of the service (e. g. on servers in different locations) and monitor the same services, or use one instance to monitor the other.
We experienced just one issue that was very annoying and a little freaky, when a server that was running both PolyMon and the SQL Server instance used by PolyMon repeatedly crashed on reboots (endless loop of reboot). Seems to be some kind of race condition. Therefore I strongly recommend to host the PolyMon service and the SQL Server service on different (virtual) machines, or set the start-up type of the PolyMon service to "Manual" instead of "Automatic", and start PolyMon manually after everything else booted, to avoid this problem.
