Home server.

Once you have built your first home server, you will wonder how you ever did without it.

What kind of servers can you have?

$ tasksel --list-tasks |  grep server
i server    Basic Ubuntu server
i openssh-server    OpenSSH server
u dns-server    DNS server
i lamp-server    LAMP server
i mail-server    Mail server
i postgresql-server    PostgreSQL database
i print-server    Print server
u samba-server    Samba file server
u tomcat-server    Tomcat Java server

That is just the tip of the iceberg.

VM Box - You can test out software or a linux distro without worrying about damaging your computer or the rest of your network!
Web Server - Host your own website for free from your home
SQL Server - Run an SQL server with your web server to store all your web-related data and stuff...
DNS Server - Run a DNS Server because its required for Windows domains. ( Hopefully I reworded it correctly)
DHCP Server - Let your server assign IP addresses to your each computer on your network.
VPN Server - When you're on vacation, or even if you're just on your smart phone, VPN into your network to instantly access to all your files! If you have your network firewalled, you can also have safe internet access on your mobile device
File Server / NAS - Who wouldn't love a giant networked hard drive that everyone in the house can share?
Print Server - Share a printer with ever computer in your house.
Computer Backup - Automagically back up all the computers in your house! If you ever have a hardware failure, don't worry! Your data is backed up on your home server
Game Server - Host a private server for your favorite game! Weather it be Minecraft, Terraria, etc.
Seedbox - If you have the bandwidth to spare, go and seed your favorite linux distros! (Or "whatever else" you want to seed )
Home Automation - Order the right hardware, and you can use your home server to automate your house! You can simpily turn off and on your lights just by speaking! There are a ton of cool things you can do with home automation.
Render Box - If you use any Auto CAD or Blender, use your server to help with rendering
Web Cache - You can install squid, and set it up as a web cache. By saving the most used resources locally, web caching can speed up web browsing.
FTP Server - Run a FTP server, and access your files from anywhere in the world! use FTPS to encrypt your FTP traffic protect your data.
Teamspeak/Mumble/Vent Server - Tired of spending money to rent a VOIP server? Just install the server for your favorite VOIP client, and you'll no longer have to pay!
Email Server - Want to get a custom domain name for your email address? Just run an email server on your home server!
Gateway - Replace your router with an IPFire server to gain access to tons of settings and features that your normal router doesn't provide.
Firewall - Protect your home network from threats by using a firewall on your gateway!
Media Server - Stream music and movies from your home server to your TV/Xbox/PS3/Computers/Other by using your home server as a media server.
Active Directory - Login to your computers, just like in a corporate network. An AD lets you manage user accounts and permissions across your whole network.
Snort Server - Run a snort server on your gateway to protect your network from malicious attacks that a normal firewall wouldn't block.
DVR (PVR) - Record Your favorite TV shows to watch later
Home Security - Record Surveillance cameras footage for home security.

FOLDING!!! - If you can't find anything better to do with your server's left over performance, Fold! It only uses the idle clocks, so it won't slow down your server!