Pi Cluster running Kubernetes
Kubernetes is a portable, extensible, open-source platform for managing containerized workloads and services, that facilitates both declarative configuration and automation. It has a large, rapidly growing ecosystem. Kubernetes services, support, and tools are widely available.
And yes, you can run your own Kubernetes cluster on Raspberry Pi's. This is a super good opportunity to elaborate with container technologies and at the same time self-host your own applications and workloads.
It's not hard to deploy this to one or several hosts. There is a maintained Ansible Playbook that will install everything automatically for you.
You can find a detailed step-by-step guide here:
Here are some examples on applications that you can host:
Nextcloud - an open-source software, first developed in 2016, that allows you to run a personal cloud storage service. It has features that are comparable to other services such as Dropbox.
Plex - gives you one place to find and access all the media that matters to you. From personal media on your own server, to free and on-demand Movies & Shows, live TV, podcasts, and web shows, to streaming music, you can enjoy it all in one app, on any device.
Radarr - is a movie collection manager for Usenet and BitTorrent users. It can monitor multiple RSS feeds for new movies and will interface with clients and indexers to grab, sort, and rename them. It can also be configured to automatically upgrade the quality of existing files in the library when a better quality format becomes available.
Bitwarden - Password manager with web-app, browser extension, and mobile app.
A list of awesome self-hosted applications here
By now, I hope you are very eager to get started. There is a very good tutorial guide here
Best of luck!!!