100% compatible Excel XLSX files.
Full Excel formatting.
Merged cells.
Defined names.
Autofilters.
Charts.
Data validation and drop down lists.
Conditional formatting.
Worksheet PNG/JPEG/GIF images.
Cell comments.
Support for adding Macros.
Memory optimization mode for writing large files.
Source code available on GitHub.
FreeBSD license.
ANSI C.
Works with GCC, Clang, Xcode, MSVC 2015, ICC, TCC, MinGW, MingGW-w64/32.
Works on Linux, FreeBSD, OpenBSD, OS X, iOS and Windows. Also works on MSYS/MSYS2 and Cygwin.
Compiles for 32 and 64 bit.
Compiles and works on big and little endian systems.
The only dependency is on zlib.

Here is an example that was used to create the spreadsheet shown above:

#include "xlsxwriter.h"

int main() {

    /* Create a new workbook and add a worksheet. */
    lxw_workbook  *workbook  = workbook_new("demo.xlsx");
    lxw_worksheet *worksheet = workbook_add_worksheet(workbook, NULL);

    /* Add a format. */
    lxw_format *format = workbook_add_format(workbook);

    /* Set the bold property for the format */
    format_set_bold(format);

    /* Change the column width for clarity. */
    worksheet_set_column(worksheet, 0, 0, 20, NULL);

    /* Write some simple text. */
    worksheet_write_string(worksheet, 0, 0, "Hello", NULL);

    /* Text with formatting. */
    worksheet_write_string(worksheet, 1, 0, "World", format);

    /* Write some numbers. */
    worksheet_write_number(worksheet, 2, 0, 123,     NULL);
    worksheet_write_number(worksheet, 3, 0, 123.456, NULL);

    /* Insert an image. */
    worksheet_insert_image(worksheet, 1, 2, "logo.png");

    workbook_close(workbook);

    return 0;
}

See the full documentation for the getting started guide, a tutorial, the main API documentation and examples.