I’ve been asked to write plugin for following scenario:
When user converts lead to contact – all attachements and notes must be copied to retrieved contact.
Code of the plugin:

using System;
using System.Collections.Generic;
using System.Text;
using Microsoft.Crm.Sdk;
using Microsoft.Crm.SdkTypeProxy;
using Microsoft.Crm.Sdk.Query;

namespace LeadConversionPlugin
{
    public class ContactCreationHandler : IPlugin
    {
        #region IPlugin Members

        public void Execute(IPluginExecutionContext context)
        {
            if (context.MessageName == MessageName.Create &&
                context.InputParameters.Contains("Target") &&
                context.InputParameters["Target"] is DynamicEntity)
            {
                DynamicEntity entity = (DynamicEntity)context.InputParameters["Target"];

                //Check that target entity is contact
                if (entity.Name != EntityName.contact.ToString())
                    return;

                //Check that this contact is created in the way of
                //convertion from lead
                if (!entity.Properties.Contains("originatingleadid"))
                    return;

                //Source Lead Identifier
                Guid leadid = ((Lookup)entity["originatingleadid"]).Value;

                //Target Contact Identifier
                Guid contactid = (Guid)context.OutputParameters["Id"];

                ICrmService crmservice = context.CreateCrmService(true);

                //Just build the query which will be used to retrieve
                //all child annotations (notes and attachements)
                QueryByAttribute query = new QueryByAttribute();
                query.ColumnSet = new AllColumns();
                query.EntityName = EntityName.annotation.ToString();

                query.Attributes = new string[] { "objectid" };
                query.Values = new object[] { leadid };

                RetrieveMultipleRequest request = new RetrieveMultipleRequest();
                request.Query = query;
                request.ReturnDynamicEntities = false;

                RetrieveMultipleResponse response = (RetrieveMultipleResponse)crmservice.Execute(request);

                foreach (annotation note in response.BusinessEntityCollection.BusinessEntities)
                {
                    //Just remove Key field of annotation
                    note.annotationid = null;

                    //Replace referencing object lookup with newly created contact lookup
                    note.objectid = new Lookup(EntityName.contact.ToString(), contactid);
                    note.objecttypecode = new EntityNameReference(EntityName.contact.ToString());

                    //And Create the annotation record
                    crmservice.Create(note);
                }
            }
        }

        #endregion IPlugin Members
    }
}

Step for this plugin must be registered as a Post Create Syncronous Parent Pipeline.