Our apartment has a key fob system to open the front door as well as an intercom system so people can call up and request to be let in. I put a Raspberry Pi Zero W inside the intercom receiver unit hooked up to the "open door" button. The Pi receives on a webhook that I control from a shortcut on my smartphone, so I can let people in even when I'm not home (or let myself in if I've forgotten the fob). The Pi also texts me whenever the doorbell rings. Not the sexiest project, but definitely the one that gets the most use!