General:
* unbelievable
* uncertain
* uncommon
* unconscious
* uncomfortable
* unfortunate
* unhappy
* unhealthy
* unimportant
* uninteresting
* unjust
* unkind
* unknown
* unlawful
* unlikely
* unnecessary
* unpleasant
* unprepared
* unrealistic
* unsafe
* unsure
* unusual
* unwanted
More specific:
* unbearable
* unbreakable
* unburdened
* unclear
* undecided
* undefeated
* uneasy
* unemployed
* unfinished
* unforgettable
* unforgiving
* unhealthy
* unimaginable
* unimpressed
* uninformed
* uninspired
* unintentional
* uninterrupted
* unjust
* unleashed
* unlock
* unmask
* unmatched
* unmentionable
* unnatural
* unpaid
* unpacked
* unprepared
* unproductive
* unqualified
* unreachable
* unrealistic
* unrecognizable
* unremarkable
* unreserved
* unresponsive
* unsolved
* unstable
* unsuccessful
* unsure
* untouched
* untrue
* unused
* unveiled
* unwilling
* unwise
* unwritten
This is just a small selection! There are many, many more words starting with "un-".