Just take a look at those that you've downloaded and used in the past. A quick look through my hard disk shows (in no particular order): iText, JFreeChart, JasperReports, HTMLUnit, jWebUnit, Stripes, the numerous Apache Commons and Jakarta projects, JExcelApi, POI, Lucene, ImageJ,
Tomcat, Apache James, DisplayTag, FlyingSaucer, PDFRenderer, HSqlDB, Derby, SquirrelSQl, iSql, Hibernate ... - and those are just the ones written in Java. (Admittedly, some of them are high-profile, too.)