There are two versions of Server Check that are completely free to use.
The free unregistered version can be downloaded, installed and used forever without requiring a license. Up to 10 monitors can be added, but the monitor types are restricted to HTTP checks and Ping checks, and the minimum time between checks is five minutes. No SMS credits are included.
The free registered version removes the five minute limit on monitor checks, so checks can be made as frequently as every 10 seconds, and all monitor types are available in this version. A small number of SMS credits are included when you first register the free version.