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 you are seeing a plug and disc on the screen like the one below go to step 2 for instructions to fix this.
If your screen is black and will not come on at all skip to step 3 and go from there.