I was curious to see what I could do with my Moodle site that could remotely accomplish what UK's Open University has done for web annotation on its own site.
After some Googling around, I think I struck gold.
Well, in case you didn't know, somewhere tucked away in a corner of the World Wide Web is a year 2008 web annotation tool for your Moodle forums - Marginalia. I have installed the Moodle plugin for Moodle 1.9 (it comes in two flavours - M2 and M1.9). In my humble opinion, the Marginalia Moodle plugin can be used by a language instructor to type corrections to his or her students' forum posts. Marginalia definitely shows it age compared to the bells-and-whistles of current, more "sexy" web annotation tools. However, it has the advantage of being built into your Moodle site once installed.
To read more, check out my Durian-inspired blog on Moodle hacks.