For Samsung Galaxy S family phones:
JIG used to get SGS phones into Download Mode so you can use something like ODIN to flash your phone back to a usable state. These are handy when your phone doesn't like the button combos to get into Download Mode OR when you've botched a flash and think you might be able to recover it once in Download Mode.
I've had this JIG save me twice already from botched flashes and when my Captivate was unresponsive to adb or button combos. All you need to do to get your phone in Download Mode with this JIG is to:
1. Remove Battery, SIM, any external SD card you may have
2. Insert Battery, do not power on
3. Insert JIG, wait a few seconds
4. You should now be in Download Mode... Unplug JIG and plug your phone into a computer with ODIN waiting.
When I ordered parts to make my JIG, I ordered a bunch of extra parts. Now that I have my JIG already, I decided to use the rest of the parts to make a few more, and sell 'em off.
For those curious, I've tested the resistance of this JIG to be 300.7 kilohms using a Fluke 179 digital multimeter.
I'm asking for $15 shipped to the United States. I have a PayPal account, so I figure that will be the easiest way to pay.
Thanks for looking!
-Andy