FOP (Formatting Objects Processor) is from the apache group. It is said to be the print formatter driven by XSL formatting objects (XSL-FO). FOP can be downloaded from http://xmlgraphics.apache.org/fop/
Its ability to convert XSL files (*.fo) to PDF certainly makes it worth mention. Basically FOP is great for template based PDF generation. Simple programs can fetch data from any data source and populate such a template, and generate PDF’s on the fly !
XSL-FO (Extensible Stylesheet Language Formatting Objects)
XSL-FO is a language for formatting XML data for output to screen, paper or other media