Grab the archive from here:

http://osdn.dl.sourceforge.net/sourceforge/freenet/freenet-0.5.1-rc2.tar.gz

:douglas:11:45:19 /v$ tar -xzvf freenet-0.5.1-rc2.tar.gz 
freenet/seednodes.ref
freenet/freenet.jar
freenet/freenet-ext.jar
freenet/README
freenet/start-freenet.sh
freenet/stop-freenet.sh
freenet/preconfig.sh
freenet/update.sh
tar: Invalid header, starting valid header search.
freenet/start-freenet.sh
freenet/stop-freenet.sh
freenet/preconfig.sh
tar: ustar vol 1, 11 files, 2088960 bytes read, 0 bytes written in 1 secs 
(2088960 bytes/sec)
tar: Failed seek on file freenet/preconfig.sh <Invalid argument>
:douglas:11:45:25 /v$

But on a FreeBSD machine:

bash-2.04$ tar -xzvf freenet-0.5.1-rc2.tar.gz 
freenet/seednodes.ref
freenet/freenet.jar
freenet/freenet-ext.jar
freenet/README
freenet/start-freenet.sh
freenet/stop-freenet.sh
freenet/preconfig.sh
freenet/update.sh
bash-2.04$

I had to re-create the archive on the freebsd machine and transfer it back to 
my NetBSD machine to read it.

Known problem? cvsweb doesn't seem to mention anything in /bin/pax.

My -current is 1.6O, i386.