JDownloader is open source, platform independent and written completely in Java. It simplifies downloading files from One-Click-Hosters like Rapidshare.com or Megaupload.com - not only for users with a premium account but also for users who don´t pay. It offers downloading in multiple paralell streams, captcha recognition, automatical file extraction and much more. Of course,

JDownloader is absolutely free of charge. Additionally, many “link encryption” sites are supported - so you just paste the “encrypted” links and JD does the rest. JDownloader can import CCF, RSDF and the new DLC files.

http://jdownloader.org/