jsPSYC is a web-based PSYC client in Javascript written by tg in 2009. It shares plenty of code with PsycZilla and is therefore available in the same source package.
jsPSYC is currently completely undocumented. It is said to require a comet engine on the server side.
Since it is distributed with PsycZilla, it is open source by triple license: Mozilla Public License (MPL), Lesser General Public License (LGPL), Creative Commons Attribution (2.5).