ASP.NET

//first let's clean up the response.object
response.Clear();			
response.ClearContent();
response.ClearHeaders();
response.Buffer = false;

//set the response mime type for excel
response.ContentType = "application/ms-excel";

response.AddHeader("Content-Disposition", string.Format("attachment;filename={0}.xls", StrFileName));
			
//create a string writer
StringWriter stringWrite = new StringWriter();

//create an htmltextwriter which uses the stringwriter
HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);

//instantiate a datagrid
DataGrid dg = new DataGrid();

//set the datagrid datasource to the dataset passed in
dg.DataSource = dr;

//bind the datagrid			
dg.DataBind();

//tell the datagrid to render itself to our htmltextwriter
dg.RenderControl(htmlWrite);

//all that's left is to output the html
response.Write(stringWrite.ToString());
response.End();