Attribute can be used on specific action or controller to override the default JSON formatting for WebApi.

CamelCaseJsonAttribute.cs

using System;
using System.Linq;
using System.Net.Http.Formatting;
using System.Web.Http.Controllers;
using Newtonsoft.Json.Serialization;

namespace DemoApplication.Extensions.WebApi
{
    public class CamelCaseJsonAttribute : Attribute, IControllerConfiguration
    {
        public void Initialize(HttpControllerSettings config,
                               HttpControllerDescriptor controllerDescriptor)
        {
            var formatter = config.Formatters.OfType<JsonMediaTypeFormatter>().SingleOrDefault();

            if (formatter == null) return;

            config.Formatters.Remove(formatter);

            // We need to new this, so it doesn't overwrite global settings
            formatter = new JsonMediaTypeFormatter
                {
                    SerializerSettings = { ContractResolver = new CamelCasePropertyNamesContractResolver() }
                };

            config.Formatters.Insert(0, formatter);
        }
    }
}