BigAtticHouse has released a small shareware package QuantumPairs LE(currently Windows-only), that allows two programmers to simultaneously edit/view/pair-program files online. You can download a free 30-day trial from http://www.bigattichouse.com/
Syntax highlighting is by file extension, and supports C/C++,PHP,HTML,Pascal,Python, and other languages.
QuantumPairs Developer Edition (scheduled for release April 15), will offer linux and windows based servers, as well as "live" version control and snapshots. We are currently developing a linux-based client, which will be released at a later date.