1. Install prerequisites

jdownloader is a great tool to download file from share websites like rapidshare.com and many more. Here are simple steps on how to install jdownloader on Ubuntu or Debian Linux distributions:
First install all prerequisites:

apt-get install openjdk-6-jre sun-java6-jre default-jre \
zenity default-jre-headles sun-java6-bin \
openjdk-6-jre-headless

for jdownloader to work correctly you must have java installed otherwise you get a following error message:

Exception in thread "main" java.lang.NoClassDefFoundError:
jd.update.Main   at java.lang.Class.initializeClass
(libgcj.so.90)Caused by: java.lang.ClassNotFoundException:
com.sun.java.swing.plaf.windows.WindowsLookAndFeel
 not found in gnu.gcj.runtime.SystemClassLoader
{urls=[file:jdupdate.jar], parent=gnu.gcj.runtime.
ExtensionClassLoader{urls=[], parent=null}}   
at java.net.URLClassLoader.findClass(libgcj.so.90)
gnu.gcj.runtime.SystemClassLoader.findClass(libgcj.so.90)
java.lang.ClassLoader.loadClass(libgcj.so.90) at java.lang.
ClassLoader.loadClass(libgcj.so.90)  at java.lang.
Class.forName(libgcj.so.90)at java.lang.Class.
initializeClass(libgcj.so.90)

2. Install Linux jdownloader

Next download jdownloader package from https://launchpad.net/~jd-team/+archive/jdownloader and install jdownloader all.deb package:

dpkg -i jdownloader_0.1-0jd1~lucid1_all.deb

During the installation you should not see any errors such us:

dpkg: dependency problems prevent configuration of jdownloader

Run Linux jdownloader

If you get dependencies errors install all depended packages. Once ready start jdownloader:

$ jdownloader

Wait for an update and you are done !