Since we are using the space on our USB drives. Is there a tool that would tell me how much free space is left on my drive? Whether its the USB drive, or the main system hard drive?
If you're using a Mac then plug the usb drive into your computer, open disk utility and the info will be displayed about any drive on the system usb or main hard drive. If you're not using a Mac, you have my sympathies ( :P) and I can't help you.
If by main system hard drive you mean the ATV then choosing Settings -> General -> About on it's menu will tell you how much free space is available.