Software for online psychological experiments that don't require users to download anything We are currently using Inquisit to conduct Internet experiments. 

Ideally, we would like to implement something that is strictly browser-based, without any extra plug-ins or downloads. We also require reaction time measurements. 

Is there anything open source out there like this?   

This question has outlined various methods that don’t fulfill all our requirements.
WebExp is a client/server based psychology/linguistics experiment creation/running system written in Java.  It is freely available.

A subject types in the appropriate web address and they see the experiment pages that have been created; obviously you have to have access to a server on which the experiment software+configuration files are running.  It supports timing and works quiet well.