For me it’s a HomeAssistant instance. Great product that has some very tangible use cases that can benefit ones household in terms of being able to implement nice automations etc, and also a great hub in that it supports such a broad range of products and services. As an Apple user in particular its one of the great ways to get non HomeKit certified devices working with Siri/Homekit on my other Apple products.
It also makes installing addons a breeze including other products people have mentioned here such as AdGuard Home (as a PiHole alternative) and the like.
A few years ago I’d say it wasn’t for the average Joe, but I think the product has really matured and is much simpler than it used to be. There’s a strong community out there too.
For multimedia I’d say Plex personally, but Jellyfin would be another option. Good way to manage personal media libraries.