1 Is the Evap system purge solenoid.
2 Is the purge hose. It goes back to the fuel system, the purge solenoid pulls fumes through it and burns them instead of venting them into the atmosphere.
3 Just hangs down, if the trans overheats and blows off fluid it will go under the truck instead of all over everything if it came out of the top vent
4 Is the crank sensor like already mentioned.
5 Is the Body control module (BCM) mostly controls interior features like door locks and such.
6 Is the Generator battery comtrol module. Basically it uses amperage draw, battery temp. etc. to regulate the battery charge voltage.
If there's anything you need more info. on let me know. I have the world of GM at my fingertips... for now