General:
* Common
* Prevalent
* Ubiquitous
* Widespread
* Pervasive
* General
* Popular
* Frequent
* Ordinary
More specific:
* Endemic (common in a particular area)
* Rife (common and widespread, often in a negative way)
* Epidemic (common and rapidly spreading, often referring to disease)
* Pandemic (common and widespread across a large geographic area, often referring to disease)
* Dominant (common and influential)
* Prevailing (common and generally accepted)
Informal:
* Everywhere
* All over the place
* A dime a dozen
The best word to use will depend on the specific context of your sentence.