Berryboot is a simple bootloader which allows the user to install multiple Linux distributions on a single SD card.  In addition it allows you to put the operating system files on an external USB hard drive instead of on the SD card itself, further expanding potential.
 
Once installed onto an SD Card, the Raspberry Pi will provide options which allow you to download and install various systems.   The main advantage of using Berry Boot over NOOBS is you can install, for example, install both Raspbian and OpenElec without having to split the available space into partition.  If you had a 32GB SD Card and wanted to install 10 different systems, each would have a less than 3GB of space to work with.  With Berryboot, all 10 systems share the available space.
 
Berry boot can be downloaded from here:  http://www.berryterminal.com/doku.php/berryboot.  
 
We also recommend visiting https://berryboot.alexgoldcheidt.com/images/.  From here you can download pre-built Berryboot images, put them onto a USB Drive, and then install those systems.  The following systems are certainly worth having a look at:
 
Volumio - Turn your Raspberry Pi into a music player which can be accessed via a web page.
Arch Linux - A non-linux streamline OS which has been used for various projects such as Recalbox.
Varios Quake builds - Yes, you can run Quake 3 on a Raspberry Pi and you can run your own server too!
Custom RetroPie builds - My be useful for those who want to emulate specific systems without going through the RetroPie setup.
Pre-build Minecraft Server images - Although there is a great guide on this website, you could try a pre-built image.  Note:  It may not be as up-to-date.
...and many many more!