I know this is an old question, but I'll add the answer just in case anyone comes here:
You can export directly to OpenOffice without customizations. The only thing you need is to go into Tools > Manage Style Sheets... and modify the existing StyleSheets so that they open OpenCalc and OpenWrite instead of Excel and Word.
Note: it's been a while since I last configured it, but I seem to remember that you might need to export and reimport the stylesheets to change the associated program.
It's quite easy, and you can actually keep both options (Export to Excel/Export to OpenCalc) so that users who need MS Office can use Excel, while the rest use OpenCalc.
This answer applies to the functionality to export to Word and Excel in Dynamics Nav 5.0 and Nav 5.0SP1. I haven't tried it in Dynamics Nav 2009 (Role Tailored Client).
You can use a library called ExcelLibrary. It's a free, open source library posted on Google Code:
ExcelLibrary
This looks to be a port of the PHP ExcelWriter that you mentioned above. It will not write to the new .xlsx format yet, but they are working on adding that functionality in.
It's very simple, small and easy to use. Plus it has a DataSetHelper that lets you use DataSets and DataTables to easily work with Excel data.
ExcelLibrary seems to still only work for the older Excel format (.xls files), but may be adding support in the future for newer 2007/2010 formats.
You can also use EPPlus, which works only for Excel 2007/2010 format files (.xlsx files). There's also NPOI which works with both.
There are a few known bugs with each library as noted in the comments. In all, EPPlus seems to be the best choice as time goes on. It seems to be more actively updated and documented as well.
Also, as noted by @АртёмЦарионов below, EPPlus has support for Pivot Tables and ExcelLibrary may have some support (Pivot table issue in ExcelLibrary)
Here are a couple links for quick reference:
ExcelLibrary - GNU Lesser GPL
EPPlus - GNU (LGPL) - No longer maintained
EPPlus 5 - Polyform Noncommercial - Starting May 2020
NPOI - Apache License
Here some example code for ExcelLibrary:
Here is an example taking data from a database and creating a workbook from it. Note that the ExcelLibrary code is the single line at the bottom:
//Create the data set and table
DataSet ds = new DataSet("New_DataSet");
DataTable dt = new DataTable("New_DataTable");
//Set the locale for each
ds.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;
dt.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;
//Open a DB connection (in this example with OleDB)
OleDbConnection con = new OleDbConnection(dbConnectionString);
con.Open();
//Create a query and fill the data table with the data from the DB
string sql = "SELECT Whatever FROM MyDBTable;";
OleDbCommand cmd = new OleDbCommand(sql, con);
OleDbDataAdapter adptr = new OleDbDataAdapter();
adptr.SelectCommand = cmd;
adptr.Fill(dt);
con.Close();
//Add the table to the data set
ds.Tables.Add(dt);
//Here's the easy part. Create the Excel worksheet from the data set
ExcelLibrary.DataSetHelper.CreateWorkbook("MyExcelFile.xls", ds);
Creating the Excel file is as easy as that. You can also manually create Excel files, but the above functionality is what really impressed me.
Best Answer
Apache OpenOffice 3.4.1 (and earlier versions) does not store in OOXML formats (.docx, .xslx, .pptx). I don't know if that will be remedied in the forthcoming Apache OpenOffice 4.0 or not.
An alternative that does have some conversion capability for OOXML in both directions is LibreOffice.