General:
* unbelievable
* uncertain
* uncommon
* unconscious
* undecided
* unemployed
* unfortunate
* unhappy
* unhealthy
* unimportant
* unlikely
* unprepared
* unrealistic
* unsure
* unusual
Negative/Opposite:
* unable
* unacceptable
* unaware
* unbreakable
* uncertain
* unchanged
* unclear
* uncomfortable
* uncontrolled
* undefeated
* unfair
* unfaithful
* unfriendly
* unhappy
* unhealthy
* unimportant
* uninterested
* unjust
* unkind
* unlawful
* unlucky
* unmarried
* unnecessary
* unpleasant
* unprepared
* unrealistic
* unsafe
* unsatisfactory
* unsuccessful
* untrue
* unused
* unwanted
* unwise
Other:
* unbutton
* unfold
* unload
* unlock
* unmask
* unpack
* unplug
* unwrap
* unsubscribe
This is just a small sample; there are many more words that start with "un."