Instafilter.io is a photo manipulation service that provides a variety of photo manipulation techniques and layering of filters. The service utilizes a RESTful API, managed by Mashape with prices beginning at $0.01/filter with the first 100/month being free and a maximum file size of 4MB.
Besides filtering photos, Instafilter.io can also edit your photos through cropping, resizing and other non-filter functions. The API is built with a mobile emphasis in mind for Android and iOS users. Instafilter.io also allows for realtime editing using HTTP requests.
A variety of client libraries are available (Python, Objective C, PHP, Java, Ruby) and with a Mashape account you can also test the API.