There are many different ways that you can respond to this question. When answering this type of question it can be helpful to understand what the interviewer is hoping to gain from your response. Keep in mind that the likely reason that the employer is asking this question is to find out whether you feel you have added value to a position and what you deem to be valuable because they understand that may play a role in how you handle the position you are interviewing for if they should hire you for that job.
The best way to handle a response to this question is to relate a situation in which you were able to accomplish something that benefited the entire organization. Remember that in posing this question the employer is not looking to gain information regarding how you have added value to your own career. Instead they are looking for information that will show them how you have been able to benefit the company by taking clear and definable action that resulted in a positive outcome.
This question is a perfect example of an interview question in which you can use a specific technique for forming an appropriate response. With this technique, you would first describe a situation that relates to the question you have been asked. The next step would be to relate the action that you took in order to resolve a problem that may have been presented to you or in order to accomplishment improvement that you saw was necessary. Finally, you would then describe the result that occurred as a result of the action that you took and how benefits were derived from that result.
When answering this type of question you do not need to go into a lot of details. It is probably better that you do not because you do not want to run the risk of beginning to ramble or getting off track in your response. Instead stick to the method that was described where you provide a situation, action taken and describe the result. This is a very simple and yet very effective way to show that you place importance on adding value to any position. Also it shows that you are willing to go beyond the call of duty and take initiative in order to provide improvement in a given situation.