Among other things:
Preservation of classic literature and texts in Arabic libraries throughout the Caliphate, containing knowledge that Europeans had lost. Among these included many works of Plato, Aristotle, and much classic Greek and Latin literature from authors like Aristophanes and Ovid. Rediscovery of these texts during the Crusades, translated from Arabic back into Italian, French, and English, were the spark that lit the fire of the European Renaissance. Were it not for Arabic librarians, there likely would not have been a Renaissance at all.
Creating a united, peaceful, and prosperous Spain while under Muslim rule. When the Muslims ruled Spain, while there was no doubt that they were in charge, there was also an atmosphere of cosmopolitan tolerance of all religions and encouragement of commerce with both Christian and Muslim lands. This helped lay the groundwork for Englightenment ideals of tolerance.
Early demonstration of the power of gunpowder as a military device; Europeans were slow to use gunpowder-based weapons until the Arabs used it to their advantage in several wars.
Innovative science and learning including many advances in astronomy and mathematics. Arabs in their Golden Age (a period of time that Europeans call the Dark Ages) invented algebra, determined the speed of light, and laid the groundwork for calculus.
