Get the current user inside a Workflow

Inside CRM 2011 Process editor it's not possible to get the current user, in some cases a reference can be necessary (specially for automated workflows) in order to set the right user when the workflow is used to create or assign records.
A Custom Workflow Activity can be created for this purpose, taking advantage of the InitiatingUserId property.

using System;
using System.Activities;
using System.Collections.Generic;
using System.Runtime.Serialization;
using Microsoft.Xrm.Sdk;
using Microsoft.Xrm.Sdk.Workflow;

namespace WFUtils
{
    public class GetCurrentUser : CodeActivity
    {
        [Output("Current User")]
        [ReferenceTarget("systemuser")]
        public OutArgument<EntityReference> CurrentUser { get; set; }
        protected override void Execute(CodeActivityContext context)
        {
            IWorkflowContext workflowContext = context.GetExtension<IWorkflowContext>();
            CurrentUser.Set(context, new EntityReference("systemuser", workflowContext.InitiatingUserId));

        }
    }
}

After the Custom Activity is deployed using the Plugin Registration Tool, a new step is available

and the output parameter (Current User) can be used inside other steps