Common words:
* jackknife
* jackpot
* jackhammer
* jackass
* jack-o'-lantern
* jackrabbit
* jackstraw
Less common words:
* jackleg (a poor workman)
* jack-in-the-box
* jack-a-dandy (a foppish man)
* jack-a-napes (a mischievous person)
* jack-tar (a sailor)
* jack-in-the-green (a traditional English May Day character)
Phrases:
* jack of all trades
* jack of hearts
* jack-in-the-pulpit (a plant)
This list is not exhaustive, but it gives you a good starting point.