CVS and firewalls
Corporate firewalls might prevent you from accessing CVS repositories directly.
If " telnet cvs.apache.org 2401" doesn't work (meaning your computer is unable to connect to port 2401 at cvs.apache.org), CVS won't work either.
This page lists known alternatives.
SFCVS which is apparently geared towards SourceForge CVS repositories. Please add a note here if you know more about it.
ProxyTunnel - ProxyTunnel is a program that connects stdin and stdout to a server somewhere on the network, through a standard HTTPS proxy. We mostly use it to tunnel SSH sessions through HTTP(S) proxies, allowing us to do many things that wouldn't be possible without ProxyTunnel.. Not tested either, please add a note here if you know more about it.
We use this at places where a proxy and firewall restricts access to CVS or other services outside the net being protected by them. The main issue with it is that you'll have a SSH daemon running in the internet that listens on port 443 (the HTTPS port) to create the tunnel.