The contiki project's CVS data is in read-only mode, so the project may have switched over to another source-code-management system. To check, visit the Project Summary Page for contiki and see if the menubar lists a newer code repository, such as SVN or Git.
The CVS data can be accessed as follows.
You can run a per-module CVS checkout via pserver protocol:
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P Attic
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P apps
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-2.x
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-apple2
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-atari
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-avr
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-c128
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-c64
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-cc65
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-coco
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-cpc
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-devel-environment
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-gb
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-gba
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-gp32
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-gtk
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-jaguar
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-msp430
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-nes
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-pcengine
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-pet
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-plus4
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-vic20
cvs -z3 -d:pserver:anonymous@a.cvs.sourceforge.net:/cvsroot/contiki co -P contiki-win32
You can view a list of files or copy all the CVS repository data via rsync (the 1st command lists the files, the 2nd copies):
rsync -a a.cvs.sourceforge.net::cvsroot/contiki/
rsync -ai a.cvs.sourceforge.net::cvsroot/contiki/ /my/local/dest/dir/
If you are a project admin for contiki, you can request that this page redirect to another repo on your project by submitting a support request.