Eversign is a secure and powerful e-Signature API that allows you to automate legally binding e-Signatures in your application using their straightforward and highly scalable JSON-based REST API. It has everything you need for legally binding digital signatures, offering an all-in-one API for creating and managing documents like contracts, NDAs, approvals, legal forms and anything else that needs a signature.
Main features include: Electronic Signatures, Documents & Security, In-Person Signing, Audit Trail, Templates, and App Integrations
This is a well documented and supported API. Pricing and plans are tiered into: 50, 200, 500 and custom number of requests.