• Wordpress written in PHP
• MediaWiki would be just another wiki among the thousand others, but Wikipedia runs on it so contributing to it will have a huge impact. It is written in PHP and uses MySQL as the database behind it. There is a separate page describing how to contribute.
• Mattermost is a messaging system, an Open Source Slack, if you wish. The back-end is written in Go, but it also has a mobile and a web client, so you have plenty of opportunities to learn and to get involved.
• Tiny RSS an rss and atom feed collector with a web interface. It is written in PHP and uses PostgreSQL as the database behind it.
• Bitwarden
• Redmine is a web-based project management application. It is written using the Ruby on Rails framework, it is cross-platform and cross-database. The only big drawback for contribution is that they are still using Subversion for version control.
• Jenkins is a web-based automation system often used to set up CI/CD pipelines. It is one of the commonly used systems in case you need an on-premise solution or for some other reason you cannot use the cloud-based CI services. It is written in Java and they have a page on how to participate.
• IntraMine indexer and search tool for all the files you have in your organization. Written in Perl. Source on GitHub

Maybe this list can help.