General Words:
* unacceptable
* unaware
* unbelievable
* uncertain
* uncomfortable
* uncommon
* unconscious
* undecided
* undefined
* unemployed
* unfair
* unfortunate
* unhappy
* unhealthy
* unimportant
* uninterested
* unkind
* unknown
* unlawful
* unlikely
* unnecessary
* unpleasant
* unprepared
* unreasonable
* unsafe
* unsatisfactory
* unsuccessful
* unusual
* unwilling
Specific Words:
* unbutton
* uncover
* undo
* unfold
* unfreeze
* unload
* unlock
* unpack
* unplug
* unroll
* untie
* unwrap
* unsubscribe
Words with "un-" negating a verb:
* unfasten (fasten)
* unlearn (learn)
* unmask (mask)
* unplug (plug)
* unscrew (screw)
* untie (tie)
This list is not exhaustive, but it gives you a good starting point. The prefix "un-" is commonly used to form the opposite of a word.