Do you have a bootable backup?
What operating system are you running? If OS X 10.6 or lower, boot from your original OS X DVD or a retail OS X DVD and then delete files as needed. Your goal should be to get to 10% of your disk capacity free.
If you have OS X 10.7 Lion or 10.8 Mountain Lion, you don't have many choices in recovery mode (boot holding the command and r keys down).
If you have a FireWire or Thunderbolt connection, you can try target disk mode with another mac.
If none of those work, perhaps your local Apple store can help with a bootable external drive.