This is what the scratch (EDIT: read scratch as * scratch * without the spaces--is there any way to escape that properly on HN?) buffer is for in Emacs, and I find it extremely useful. Also, unlike using a different tool, it allows me to use all the Emacs-specific features I usually rely on. (For example, I can easily type special characters like r₁ × r₂ ≈ r₃ using the TeX mode.)

If you want more than one scratch buffer--which happens to me once in a while--you can just create a new buffer with any name, and it will also do. New buffers are in a different mode by default, but you can set it up to work exactly the same way as scratch if you want.

As another commenter pointed out, you can use the browser to evaluate JavaScript. Emacs lets you do the same thing with elisp in the scratch buffer by default: try entering in an elisp expression and pressing C-j.

Just a fun alternative to this trick for the Emacs users around here :).