Here's how to make a bootable (or not) USB stick from a disk image in OSX using Disk Utility found under: Applications/Utilities/

You can use this cloning method to make a bootable Mac OS X install USB stick or an ASD one. You could also use this method to clone a small hard drive or any volume you want, given that it fits on the size of your USB drive.  If the source volume that you want to clone already contains a System Folder that's appropriate for your Mac, then the resulting USB stick could be used to boot your Mac by inserting it in an USB port and rebooting while pressing down the OPT (ALT) key until you see the startup manager screen.

Note: This article refers to "USB stick" but it could be any kind of external drive.

1) Grab a random 8GB or larger USB stick (or USB hard drive) and copy whatever you want to keep from it onto your computer's hard drive, because we're going to completely erase everything on that USB stick.

2) On the desktop, mount (just double click) the disk image (e.g. Snow_Leopard_10_6_3.dmg) that you want to clone.

3) Launch Disk Utility.

4) Click the root of your USB stick on the left panel (often named as the drive brand/model), click ERASE at the top, leave NAME as "Untitled", select "Mac OS Extended (Journaled)" from the FORMAT dropdown menu.  Before you click ERASE, if it asks you for a SCHEME, choose accordingly:

• If the drive is destined to a PowerPC (G3, G4, G5) it requires APPLE PARTITION MAP (APM)
• If the drive is destined to any other Mac (Intel, ARM) it requires GUID.
• If the drive is destined to Windows, then it requires Master Boot Record (MBR).

5) Click the "Untitled" item immediately below the root of your USB stick (2nd element) on the left panel and click RESTORE at the top.  Note that on some Intel OSX versions, RESTORE is found in the EDIT menu.

6) On PowerPC OSX, you could simply drag and drop the mounted disk image's partition (2nd element) from the left panel to the SOURCE box on the right panel and then drag and drop your USB stick's partition (the 2nd element) from the left panel onto the DESTINATION box on the right panel. On Intel OSX, they removed that feature and replaced it with a dialog window that asks you to select "RESTORE FROM" so just select the mounted volume that you want copy from.

7) Click RESTORE.