From a list of SObject and a field Api Name returns a Map with a list of SObject per field value

SObjectService.cls

/*
	* Example: Get a map of list of contacts by status from list of contacts
	* @param: List<SObject> objects
	* @param: String fieldName - field Api Name of a text field
	* @return Map<String, List<SObject>> sObjectListByString
	*/
	public static Map<String, List<SObject>> getSObjectListByString(List<SObject> objectList, String fieldName) {

		Map<String, List<SObject>> sObjectListByString = new Map<String, List<SObject>>();
		String fieldValue;

		for (SObject obj: objectList) {
			fieldValue = (String) obj.get(fieldName);
			if (sObjectListByString.containsKey(fieldName)) {
				sObjectListByString.put(fieldValue, new List<SObject>());
			}
			sObjectListByString.get(fieldValue).add(obj);
		}

		return sObjectListByString;

	}