Similar in meaning, but slightly less harsh:
* Cowardly
* Timid
* Fearful
* Spineless
* Pusillanimous
More extreme synonyms:
* Dastardly (implies treachery or wickedness)
* Vile (implies extreme moral depravity)
* Despicable (implies extreme worthlessness)
The best synonym will depend on the specific context in which you want to use it.