APIs [My Pick!]

Apache POI [My Pick!]
Java API To Access Microsoft Format Files
http://poi.apache.org

Apache Commons [My Pick!]
The Commons is an Apache project focused on all aspects of reusable Java components.
http://commons.apache.org/

JCom (Java-COM Bridge) [My Pick!]
JCom is a bridge library between Java and Com to enable COM object access from Java classes. Example: Creating an Excel worksheet, or using Visual Basic COM application from a Java application.
http://sourceforge.net/projects/jcom