I have grown quite dependent on my home server. Some of the uses were not obvious at first.
It provides long term data storage for big files that I don't want on my desktop.
Automated scripts use it as a destination for backups.
Bittorrent downloads/seeds are out of sight, out of mind.
SSH gives me a safe way to access it from the outside.
GNUMP3D uses HTTP to stream my music collection to any computer.
Clone and restore computers via the network using Clonezilla.
Compile programs from source without slowing down your desktop.
NFS works great with Linux boxes, but I also keep samba running so windows machines can access it.
That 15K SCSI drive is great for performance. Complement it with some big storage drives. I prefer the slower, but cool running and energy efficient 5400 RPM SATA drives for that purpose.
There are several more possibilities. Give the members of this forum a little time and I'm sure a lot more good ideas will be posted.