The AJAX Search API is available for sites that are accessible to their end users without charge. You can use the API on both commercial and non-profit websites as long as your site meets the terms of use.
Please bear in mind that Google can't officially "sign off" on every website that uses this API, so it's your responsibility to make sure your site meets the terms of use before you develop and launch it. Google reserves the right to suspend or terminate your use of the service at any time, so please read the terms of use carefully.