Ubuntu 14.04 AMD64 bootable USB drive.
I created mine using the StartupDiskCreator creator tool included with Ubuntu after downloading the latest .iso from ubuntu.com.
Step 1: Install rEFInd.Link here: http://www.rodsbooks.com/refind/
Make sure it persists across reboots before moving on to next step.
Step 2: Free up some space on the hard drive.With Yosemite I had to revert from a logical volume to a normal partition in order to be able to resize the OSX partition. On the command line run this.
diskutil cs list
You'll see your corestorage devices listed. One entry, the last one to print, will have this tag or something like it: "Revertible: Yes"
Take the UUID from this entry and revert it by running a command like this:
diskutil cs revert 7BF42B7B-xxxx-xxxx-xxxx-xxxxxxxxxxxx
Shut down completely and then power on the computer. Not sure why this is necessary, but it is.
You can now use the OSX disk tool to resize the partition by dragging the corner. Leave the rest of the space empty.
Step 3: Install UbuntuPlug in the USB stick. When you shutdown and restart the rEFInd will have it as an option to boot into.
Step through the install screens as normal and choose to "do something else" during the disk selection step.
Add two partitons, one ext4 for root (/) and one as swap.
This is important: make sure to install the bootloader on the same partition you chose for root (/). This is available as a dropdown menu option on the partitioning screen.
Finish the install, the next time you reboot you'll see the OSX logo and a Penguin in your rEFInd boot menu. That's it.
To get the computer to sleep properly, install the proprietary gpu drivers. This is a one click install under the "additional drivers" tool.
To right click, use two fingers while clicking.
I've found it useful to disable tap-to-click on the touchpad. This checkbox is in the mouse settings tool.