I agree that Open or Negotiable are good answers if you can use them on a form ... If you're applying online, many sites these days require you to enter a numeric salary requirement or select a range from a list. I find Salary.com
useful in those situations (for jobs in the USA), since you can enter the job title and your zip code and it will return a salary distribution curve. Specifying a salary near the mean for your area is a safe bet in most cases (go low if you have little experience, go high if you have a lot of experience or special skills).