Comparison of Java APIs for Excel

The Aspose folks have put up a comparison of their Java Excel API vs popular open source APIs such as JExcelAPI and Apache POI.

The comparison is worth a persual:
http://www.aspose.com/docs/display/cellsjava/Aspose.Cells+for+Java+Vs+Open+Source+Competitors

Open source API for read/write to Excel files

Long back, I had blogged about native APIs in .NET and Java to read/write Excel files.
Recently came across a new native .NET library that can be used to read/write Excel files and supports the binary BIFF format. This is the format used by Excel 2005 files - i.e. xls files. This API is still evolving and looks quite basic at this point of time. 
The code is available here:  http://code.google.com/p/excellibrary/
There is also a .NET port of the popular Apache POI library available at: http://code.google.com/p/npoi/

The new office (2007, 2010) documents  are based on XML standards (i.e. xlsx files). There are a couple of open source projects for creating and modifying 2007 'xlsx' excel files.
1) http://epplus.codeplex.com/
2) http://excelpackage.codeplex.com/