If you used a shared-hosting service like me, often times they allow you to host an unlimited number of subdomains. I've seen a lot of web developers let this go to waste; there is a huge opportunity here to brand yourself or your organization using your subdomains.
The way I take advantage of unlimited subdomains is by linking to all of my social media websites through my domain name. For example, I have set the following up:
These are all sites that I commonly refer people to, or that I frequently use. Each of these is linked to my profiles/pages on those pages.
If you wanted, you could even take a step above and log the stats on these domains. Instead of using a redirect through your host, you could setup a hosted subdomain and log stats on how many people visit your social media pages; from here, you can collect rough-stats that you would otherwise not have (since most social media sites don't allow you to view stats on your pages).
If anyone has any other suggestions for subdomain uses, leave a comment below.