I am looking for a robust open source tool for monitoring SQL server. Ours is a small organization and we don't have any monitoring tool in place and would like to know if any open source tool is available so that we can start monitoring lower environments to start with.
Stack Exchange offers their monitoring tool as open source. It's really powerful, if you can get over the installation process.
We use Zabbix on several servers, it's open source and you can do a lot of things. http://www.zabbix.com After installing you have to setup a SQL Server template, it contains items to monitor. The zabbix website has some templates for download https://share.zabbix.com/databases/microsoft-sql-server.
Hope this helps.
Below is a link to SQL Server tools including monitoring.
Some are open source, others paid. Review what is needed, and adapt using tools and reports. Also remember sql agent can provide alerts for key items (failed backup, room, fragmentation, etc.).