Write notes or chat about Stack Overflow questions and answers directly on the question page.
Questions, answers, and comments on Stack Overflow are carefully curated for concise, quality content. "Chattiness" is explicitly discouraged, reserved for actual SO chatrooms. But I'd rather have something more informal than comments on the question page itself for the occasional note to self, back and forth, or snarky diatribe.
That pretty much sums up Stack Chat. When you click the browser icon, a panel slides out with messages relevant to the question page you are on. If you aren't on a question page, you'll get some global chats to peruse. If you aren't on Stack Overflow at all, it will redirect you there.