ExcelToDownloadable.cs

public static DownloadableFile ToDownloadableXmlFileForExcel2003(this System.Xml.Linq.XDocument file, string fileName)
{
    MemoryStream ms = new MemoryStream();

    XmlWriterSettings xmlWriterSettings = new XmlWriterSettings() { Encoding = Encoding.UTF8 };
    XmlWriter xmlWriter = XmlWriter.Create(ms, xmlWriterSettings);

    file.Save(xmlWriter);   //.Save() adds the <xml /> header tag!
    xmlWriter.Close();      //Must close the writer to dump it's content its output (the memory stream)

    DownloadableFile dbf = 
            new DownloadableFile
            {
                FileName = String.Format("{0}.xls", fileName.Replace(" ", "")),
                Content  = ms.ToArray(),
                MimeType = "application/vnd.ms-excel"
            };

    ms.Close();
    ms.Dispose();

    return dbf;
}

IEnumerableToExcel.cs

sing System;
using System.Collections.Generic;
using System.Linq;
using System.Xml.Linq;

namespace YourNameSpace
{
    public static class ExcelExportExtensions
    {
        public static XDocument ToExcelXml(this IEnumerable<object> rows)
        {
            return rows.ToExcelXml("Sheet1");
        }

        public static XDocument ToExcelXml(this IEnumerable<object> rows, string sheetName)
        {
            sheetName = sheetName.Replace("/", "-");
            sheetName = sheetName.Replace("\\", "-");

            XNamespace mainNamespace = "urn:schemas-microsoft-com:office:spreadsheet";
            XNamespace o = "urn:schemas-microsoft-com:office:office";
            XNamespace x = "urn:schemas-microsoft-com:office:excel";
            XNamespace ss = "urn:schemas-microsoft-com:office:spreadsheet";
            XNamespace html = "http://www.w3.org/TR/REC-html40";

            XDocument xdoc = new XDocument(new XDeclaration("1.0", "utf-8", "yes"));

            var headerRow = from p in rows.First().GetType().GetProperties()
                            select new XElement(mainNamespace + "Cell",
                                new XElement(mainNamespace + "Data",
                                    new XAttribute(ss + "Type", "String"), p.Name)); //Generate header using reflection

            XElement workbook = new XElement(mainNamespace + "Workbook",
                new XAttribute(XNamespace.Xmlns + "html", html),
                new XAttribute(XName.Get("ss", "http://www.w3.org/2000/xmlns/"), ss),
                new XAttribute(XName.Get("o", "http://www.w3.org/2000/xmlns/"), o),
                new XAttribute(XName.Get("x", "http://www.w3.org/2000/xmlns/"), x),
                new XAttribute(XName.Get("xmlns", ""), mainNamespace),
                new XElement(o + "DocumentProperties",
                        new XAttribute(XName.Get("xmlns", ""), o),
                        new XElement(o + "Author", "Smartdesk Systems Ltd"),
                        new XElement(o + "LastAuthor", "Smartdesk Systems Ltd"),
                        new XElement(o + "Created", DateTime.Now.ToString())
                    ), //end document properties
                new XElement(x + "ExcelWorkbook",
                        new XAttribute(XName.Get("xmlns", ""), x),
                        new XElement(x + "WindowHeight", 12750),
                        new XElement(x + "WindowWidth", 24855),
                        new XElement(x + "WindowTopX", 240),
                        new XElement(x + "WindowTopY", 75),
                        new XElement(x + "ProtectStructure", "False"),
                        new XElement(x + "ProtectWindows", "False")
                    ), //end ExcelWorkbook
                new XElement(mainNamespace + "Styles",
                        new XElement(mainNamespace + "Style",
                            new XAttribute(ss + "ID", "Default"),
                            new XAttribute(ss + "Name", "Normal"),
                            new XElement(mainNamespace + "Alignment",
                                new XAttribute(ss + "Vertical", "Bottom")
                            ),
                            new XElement(mainNamespace + "Borders"),
                            new XElement(mainNamespace + "Font",
                                new XAttribute(ss + "FontName", "Calibri"),
                                new XAttribute(x + "Family", "Swiss"),
                                new XAttribute(ss + "Size", "11"),
                                new XAttribute(ss + "Color", "#000000")
                            ),
                            new XElement(mainNamespace + "Interior"),
                            new XElement(mainNamespace + "NumberFormat"),
                            new XElement(mainNamespace + "Protection")
                        ),
                        new XElement(mainNamespace + "Style",
                            new XAttribute(ss + "ID", "Header"),
                            new XElement(mainNamespace + "Font",
                                new XAttribute(ss + "FontName", "Calibri"),
                                new XAttribute(x + "Family", "Swiss"),
                                new XAttribute(ss + "Size", "11"),
                                new XAttribute(ss + "Color", "#000000"),
                                new XAttribute(ss + "Bold", "1")
                            )
                        )
                    ), // close styles
                    new XElement(mainNamespace + "Worksheet",
                        new XAttribute(ss + "Name", sheetName /* Sheet name */),
                        new XElement(mainNamespace + "Table",
                            new XAttribute(ss + "ExpandedColumnCount", headerRow.Count()),
                            new XAttribute(ss + "ExpandedRowCount", rows.Count() + 1),
                            new XAttribute(x + "FullColumns", 1),
                            new XAttribute(x + "FullRows", 1),
                            new XAttribute(ss + "DefaultRowHeight", 15),
                            new XElement(mainNamespace + "Column",
                                new XAttribute(ss + "Width", 81)
                            ),
                            new XElement(mainNamespace + "Row", new XAttribute(ss + "StyleID", "Header"), headerRow),
                            from contentRow in rows
                            select new XElement(mainNamespace + "Row",
                                new XAttribute(ss + "StyleID", "Default"),
                                    from p in contentRow.GetType().GetProperties()
                                    select new XElement(mainNamespace + "Cell",
                                         new XElement(mainNamespace + "Data", new XAttribute(ss + "Type", "String"), p.GetValue(contentRow, null))) /* Build cells using reflection */ )
                        ), //close table
                        new XElement(x + "WorksheetOptions",
                            new XAttribute(XName.Get("xmlns", ""), x),
                            new XElement(x + "PageSetup",
                                new XElement(x + "Header",
                                    new XAttribute(x + "Margin", "0.3")
                                ),
                                new XElement(x + "Footer",
                                    new XAttribute(x + "Margin", "0.3")
                                ),
                                new XElement(x + "PageMargins",
                                    new XAttribute(x + "Bottom", "0.75"),
                                    new XAttribute(x + "Left", "0.7"),
                                    new XAttribute(x + "Right", "0.7"),
                                    new XAttribute(x + "Top", "0.75")
                                )
                            ),
                            new XElement(x + "Print",
                                new XElement(x + "ValidPrinterInfo"),
                                new XElement(x + "HorizontalResolution", 600),
                                new XElement(x + "VerticalResolution", 600)
                            ),
                            new XElement(x + "Selected"),
                            new XElement(x + "Panes",
                                new XElement(x + "Pane",
                                    new XElement(x + "Number", 3),
                                    new XElement(x + "ActiveRow", 1),
                                    new XElement(x + "ActiveCol", 0)
                                )
                            ),
                            new XElement(x + "ProtectObjects", "False"),
                            new XElement(x + "ProtectScenarios", "False")
                        ) // close worksheet options
                    ) // close Worksheet
                );

            xdoc.Add(workbook);

            return xdoc;
        }
    }
}