Common words:
* Un (a/an)
* Une (a/an, feminine)
* Unique (unique)
* Urgent (urgent)
* Utiliser (to use)
* Usine (factory)
* Université (university)
* Unité (unity)
* Usage (usage)
* Utile (useful)
Less common words:
* Ubac (north-facing slope)
* Ubiquité (ubiquity)
* Uchronie (alternate history)
* Ultra (extreme)
* Uniforme (uniform)
* Univers (universe)
* Uranium (uranium)
* Urne (urn)
* Usager (user)
* Ustensile (utensil)
Nouns:
* Unité (unit)
* Usine (factory)
* Université (university)
* Usage (use)
* Ustensile (utensil)
* Urne (urn)
Verbs:
* Utiliser (to use)
* Unir (to unite)
Adjectives:
* Unique (unique)
* Urgent (urgent)
* Utile (useful)
Pronouns:
* Un (a/an, masculine)
* Une (a/an, feminine)
This is not an exhaustive list, but it gives you a good starting point for French words that begin with the letter "U". Remember that the "U" can be pronounced differently depending on the word and context.