According to ReligionFacts, the most prominent symbol in Islam is the star and crescent. It is often on a green background and is heavily used by other countries in the past and present.
Wikipedia notes that prior to the star and crescent, followers of the Prophet Muhammad waved solid-colored flags with no symbols for identification purposes. In 1844, the Ottoman Empire incorporated the symbols on their flag, which modern-day Turkey still uses as the national flag. Other countries, such as Tunisia and Algeria, included the symbols during the 1950s. More recently, Libya began using the symbol following the 2011 protests.