There are quite a few useful things you can do with a server (Mac or *nix).
1. Make it a Squid server. See:
http://www.squid-cache.org/
A Squid server in my opinion is the most useful thing you can do for your brothers... there is virtually nothing to teach them on how to use it. And it will make all their websurfing easier and faster.
2. A webserver. Depending on how the campus (or your house) is connected you can set up a webserver and host students and the fraternity's "personal" page(s). Pretty handy but kinda redundant if the school already supplies those resources.
3. Database server. MySQL/PHP/Apache... all kinds of useful stuff can be done with this... the sky is NOT the limit... your imagination is.
4. Samba file server. Very useful IF anyone is interested in keeping or sharing files from the server (MP3s, MPEGs, WAVs, whatevers). As a repository for common downloads, as a backup for their "My Documents" directory. If you store other peoples files on your server you better have a damn good backup plan.
-DU-...etc...