WCF error : 405 Method Not Allowed

You need to use JSONP for a cross-domain call to get round the browser restrictions, and to update your web.config with crossDomainScriptAccessEnabled set to true to get round server ones. There’s a good example in the answer here: how to avoid cross domain policy in jquery ajax for consuming wcf service?

You may also have a problem with GET requests. Try the fixes outlined here:
Making a WCF Web Service work with GET requests

Altogether, you want a web.config that looks something like this:

<bindings>
  <webHttpBinding>
    <binding name="crossDomain" crossDomainScriptAccessEnabled="true" />
  </webHttpBinding>
</bindings>
<behaviors>
  <endpointBehavior>
    <behavior name="restBehavior">
      <webHttp />
    </behavior>
  </endpointBehavior>
  <serviceBehavior>         
     <behavior name="MyServiceBehavior">
        <serviceMetadata httpGetEnabled="true"  />
        <serviceDebug includeExceptionDetailInFaults="true"/>
     </behavior>
  </serviceBehavior>
</behaviors>
<services>
  <service name="..." behaviorConfiguration="MyServiceBehavior">
    <endpoint address="" binding="webHttpBinding" bindingConfiguration="crossDomain" 
              contract="..." behaviorConfigurations="restBehavior" /> 
  </service>
</services>

(Note that both the service and the endpoint have behaviours attached, allowing webHttp calls and httpGet calls respectively, and that the binding has crossDomain access explicitly enabled).

… a service method decorated like this:

[ServiceContract]
public interface IMyService
{
    [WebGet] // Required Attribute to allow GET
    [OperationContract]
    string MyMethod(string MyParam);
}

… and a client call using JSONP:

<script type="text/javascript">
$(document).ready(function() {
    var url =  "...";
    $.getJSON(url + "?callback=?", null, function(result) { // Note crucial ?callback=?
       // Process result
    });
});
</script>