The download link works for me. Maybe the mirror server was offline?
Thanks Wolfang! already works for me too.
You should have started from here: http://www.jboss.org/jbossas/downloads/
For 5.0.0.GA, you should get to here: http://sourceforge.net/project/showfiles.php?group_id=22866&package_id=16942&release_id=645033
The *.MD5 and *.SHA-256 files are hashes that you can use to verify the downloads.
The jboss-5.0.0.GA.zip file is the binary for use with JDK 5.
The jboss-5.0.0.GA-jdk6.zip file is the binary for use with JDK 6.
The jboss-5.0.0.GA-src.tar.gz file is the source for the app server (not the full source, though, many other projects appear only as JAR files).
Thx Peter, it's working now.