Recently i was brought an iPod Touch 3g 8Gb to fix. The issue? This iPod was running 3.1.2 firmware and was jailbroken and under some unknown circumstance ended up bricked (I say unknown because it is not mine). Normally when a 3g is jailbroken it is teathered, meaning that if turned off it must be connected to the computer and re-jailbroken to turn it back on. This could be a big number of the problems that drew you to this instructable so i will cover that as well.
Note: I have only performed this on the aforementioned iPod so i do not know if this works on others.
Step 1: Tell the difference between bricked and tethered.
If your screen is black and will not come on at all skip to step 3 and go from there.
Step 2: Tethered
First off if you have a jailbroken iPod and you turn it back on you may see a screen with a plug and a disc like the picture below. This does not mean that your iPod is "bricked". the fix to this is actually quite easy. Just plug it into the computer and run thew software that you used to jailbreak it. For example, I use Blackra1n so i would plug in the ipod and wait for it to be recognized and then click the button thats says "make it rain" click okay at the popup window and wait.
Step 3: Bricked
This was a tricky fix as i could not find much info on it. The iPod was black and would not turn on, show the tethered icon, go into recovery mode or DFU mode. When i plugged it into itunes and tried restoring i got a error everytime. If you are having the same problem move to the next step.