General synonyms:
* Promised
* Vowed
* Committed
* Guaranteed
* Assured
More specific synonyms depending on context:
* For a commitment: Dedicated, devoted, consecrated
* For a financial promise: Guaranteed, secured, insured
* For a political promise: Committed, bound, obligated
* For a romantic promise: Sworn, betrothed, promised
It's important to consider the specific context in which "pledged" is used to choose the most accurate synonym.