direct access to the browsers javascript console
No. Not without opening the console manually, and also you cannot do much more than logging messages. Any emulator will not use the native console.
web-based front end console at CodeAcademy.com
Have you looked at their source code? You can write such one yourself, too. Or have a look at EloquentJavaScript's console script (using Mochi and Codemirror).
a browser console
…is not possible without only emulating it. However, you can use FirebugLite for that, which promises the same look-and-feel as the native Firebug. Also, Opera's Dragonfly is written in JS, and it is released as open source so you might adapt (parts of) it.