GemBox.Spreadsheet
.NET Excel Library

static void Main()
{
    // Set license key to use GemBox.Spreadsheet in Free mode.
    SpreadsheetInfo.SetLicense("FREE-LIMITED-KEY");

    // Create a new empty Excel file.
    var workbook = new ExcelFile();

    // Create a new worksheet and set cell A1 value to 'Hello world!'.
    workbook.Worksheets.Add("Sheet 1").Cells["A1"].Value = "Hello world!";

    // Save to XLSX file.
    workbook.Save("Spreadsheet.xlsx");
}