PPP over SSH
WHY?????? WHY NOT? 
Run this on client:
pppd noauth debug nodetach 10.0.0.1:10.0.0.2 pty 'ssh user@server "pppd noauth debug nodetach notty "'
It works, even if the server asks for password, as it uses a direct terminal I/O, not stdin. Note that for noauth option pppd needs to be run as root (otherwise, some authentication options have to be set).
BTW Chris found this link. I think our one-liner is much nicer