I had a problem with a server at work recently. I used to use a hint from the macosxhints site to reverse ssh into my home mac. However, I can't do that anymore because my mac is behind NAT. What I needed was a way to connect to the mac at work which was behind a firewall (but not nat). I had another machine running FreeBSD with SSH open.
Connect to machine one
ssh -C -4 -L 5901:127.0.0.1:5900 remotehostyouwant Connect to work mac:
ssh -C -4 -L 5900:127.0.0.1:5900 macatwork