Just about anything! Anything that calls for a LAMP stack should work. Plain HTML, various scripting languages are also usable via FastCGI, PHP 7.4 and 8.0 are supported. If WordPress is your thing; Plesk’s WordPress modules are enabled by default to make installing and updating it easy; and a number of other items are available via the Plesk panel’s Applications section for you to browse and install. Another available item is Nextcloud, which is a great alternative to other cloud services for storing and syncing files, calendars, contacts, etc.
Please note, the application installers provided by Plesk are not specifically reviewed or managed by Lower Ground, they’re just a handy feature of the hosting panel we make available to you. You are still responsible for properly managing the security and configuration of the websites you host.