General:
* Ordinary
* Commonplace
* Average
* Typical
* Normal
* Unremarkable
* Mediocre
More specific:
* Mundane (everyday, lacking excitement)
* Banal (unoriginal, trite)
* Dull (lacking interest or excitement)
* Mediocre (of moderate quality)
* Unremarkable (not worthy of notice)
The best antonym will depend on the context in which "extraordinary" is used.