I use mine as a server. A desktop needs to compute a lot of graphics, a server does typically not, so it can handle quite a large load !

Moreover, it teach one to manage limited resources: if something is slow for my low-traffic RPi, it will not scale well.

- Web server: *httpd with a static website generator. Or even better: /gopher/ server!

- Streaming service: mpd, web-based

- Media box: plugged to a TV accessing content from all computers in the house.

- NAS: ditch $CORP-Drive free offer and get gour data $HOME. ;) You can buy a couple of USB-to-SATA cables and plug multiple hard drives, possibly with ZFS. Slow? Not a big deal, yournetwork may be slower anyway (for me it is).

- SSH access for a remote shell, you can even give friends access to your shell, as it will teach you user, groups and privileges management. You can also do Linux containers and BSD jails.

- Download station: run long (legal ofc :P) downloads queues without fearing the need of turning your laptop off. This plays nicely with media box / NAS storage.

- Filter spam and sort the mail. Either with a full mail server (run your own mails), with a client (retrieving the mail, sorting them and then syncing them back (isync...)), or both (a client retrieve the mail, you sort them, then a server serve it to you, but still using smpt.your-mail-provider.com to send mail to avoid being considered as spam).

- Hate to miss what's going on on a channel? IRC logs or bouncer.

- Using git ? A git repo stored on a server already acts as a git server, even if you do not have git installed on a server!

- Pfew! Many things to start and monitor... You may want to learn how to use your service manager (service ssh start....).

- You start to have a lot of services running (yes, run them ALL). Let's monitor their performances: *top is fine for interactive use, but you will need statistics for servers (how many times a week a service is used). collectd can help: plugin for anything in, and multiple output formats, such as Round Robin Databases (err, this is binary and not plain text), or good ol' csv.

- Ok we got a problem: one service is used everyday and all of the other are inactive, but still listenning for incoming connection on their own, wasting resources ( >:| my precious resources !). Let's use inetd to which is one program that listen on all specified ports, shut down all underused services, and let inetd call appropriate service while a client ask for it. You now have only one daemon listenning on all ports.

I'm sure a Raspberry or similar board can handle it all. It wilk only have to run most of these services once in a while, and not all at once.

If you start to feel it to be slow, you can get another one. IIRC servers do not need high frequency CPU but many-cores CPU. You can buy more boards and spread services across them all. You will then have your own private network! Now you can have fun with routing, and manage certificates to secure traffic across them.