Most monitoring tools are not designed specifically to monitor MySQL servers. Instead, they are general-purpose systems designed to periodically check the status of many kinds of resources, from machines to routers to software (such as MySQL). There are dozens of monitoring tools (free or commercial) to choose from, making it a very time-intensive process to evaluate the offerings and decide whether a specific one suits you well. Here are some of the most popular free monitoring tools you may like:
Nagios is probably the most popular open source problem detection and alerting system. It periodically checks services you define and compares the results to default or explicit thresholds.If the results are outside the limits, Nagios can execute a program and/or alert someone to the trouble.
Nagios can run any executable file as a plugin (based on plugin architecture), provided it accepts the right arguments and gives the right output. As a result, Nagios plugins exist in many languages, including the shell, Perl, and other scripting languages. And if you can't find a plugin that does exactly what you need, it's simple to create your own. A plugin just needs to accept standard arguments, exit with an appropriate status, and optionally print output for Nagios to capture.
Zabbix is the ultimate enterprise-level software designed for monitoring availability and performance of IT infrastructure components. With Zabbix it is possible to gather virtually limitless types of data from the network. Along with storing the data, visualization features are available (overviews, maps, graphs, screens, etc), as well as very flexible ways of analyzing the data for the purpose of alerting.
OpenNMS is an award winning network management application platform with a long track record of providing solutions for enterprises and carriers.