We were a bit surprised to not find a REST API.

We were wanting to do something as simple as updating a custom field for a client remotely but were unable to do this.

Please consider implementing this in the future. Most software these days has this as it is essential for developers wanting to do integration.

We have an XML-RPC API presently, however we will likely look at switching that over to REST in 4.0. When it was implemented, REST and XML-RPC were both about equivalent in usage, however the simplicity of REST has since seen its usage surpass XML-RPC.