Creating a Windows 8 bootable usb on Mac 10.6.8 without BootCamp?
None of the answers got it right.
This is how you do it — without BootCamp:


Get/download a Windows ISO image file.
Your USB flash drive should have at least 8GB.
Plug it in and format/erase it using Disk Utility (under Applications/Utilities)


CAUTION: Make sure you select the correct drive! Then click erase.
Select MS-DOS (FAT) for the Format.
(If your drive is above 32GB, you might need to select ExFAT instead.)
Select Master Boot Record for the Scheme.
(This is the one used for Windows partitions.)
Click erase.


Open Terminal and run command: diskutil list
From the output, find out the name of the USB drive. It should be something like /dev/diskX. X is the number of the drive. For me, it's /dev/disk2. It might be different on your Mac.
Then type command: diskutil unmountDisk /dev/disk2
It should print something like this: Unmount of all volumes on disk2 was successful.
Then type:
sudo dd if=/path/to/win-iso-file of=/dev/disk2 bs=1m
(This will ask for your Mac password. Enter it and hit return.)


/path/to/win-iso-file is the full path of the ISO image. Just drag/drop it on terminal after if= and it will print the full path. 

The process will start but there will be no output for a while. It might take 10 to 45 minutes. Be patient! (You can press Ctrl+T to see the bytes transferred so far.)
Once it's complete, it will output records in/out and bytes transferred. 
Type: diskutil eject /dev/disk2
Now, re-plug the USB drive and check its contents. It should have files and folders but especially a setup.exe and an autorun.inf file.

Done.