Stardestroyer.net fits that description somewhat, for values of "casually" that allow for copious swearing punctuating most disagreements. I haven't posted there, but Kaj Sotala posts as Xuenay (apologies no apologies for stalking). Examples of threads on LW-related topics: * Logic Fallacy reference thread Mk. 2 * The Singularity in Sci-Fi (haven't read) * Robots Learn How to Lie * Mini-FAQ on Artificial Intelligence (the thread that properly introduced me to SIAI) * This derailment about Three Worlds Collide * Rationality (short) * And the Winner for Most Probable God is... Azathoth? * Transhumanism: is it viable? (Edited after first upvote; later edited again to add a link.)

Pharyngula. More atheist than rational, and more biology than technology, but it is definitely a community. It is a blog, but has an interesting feature called the endless thread which is kind of "collective stream of consciousness". Check it out. And also look at other offerings in the science blogosphere. [Edit:supplied link.]