JDownloader is a Download Manager written in Java. JDownloader can download normal files, but also files from online file hosting services like Rapidshare.com.
For running JDownloader you need Java installed. I recommend OpenJDK, it works flawless with jDownloader.
# pacman -S openjdk6
You can use the jdownloader package in the AUR.
Use the command jdownloader to start JDownloader. When you just installed JDownloader from AUR, this will run the update tool to download some required files for JDownloader, else this will start JDownloader directly.
When you first start JDownloader you can choose your preferred language and also your download directory. On the next window, JDownloader asks you if you want to install FlashGot, a Firefox extension. I recommend clicking on Cancel. If you want this extension, you can still download it through the official mozilla addons website.
Tips and tricks
Making JDownloader faster
In its default configuration, JDownloader is very slow and uses a lot of resources. You can atleast make it a bit faster by applying the following changes in the Settings Tab.
Choose General and then turn the logging level to OFF.
Choose User Interface and then switch the style to Light(GTK). (If you're using GNOME).