Virtual Classrooms are probably the theme best developed in the Educational MOO community. Based on earlier work at Lambda-MOO and other almost mythical MOOs, a certain number of people like Ken Schweller or DaveM  have developed very interesting artifacts for teaching and tutoring in the MOO. Most of those objects can be found on DU 5.1.1 but many have also found their way into other MOOs. Also, consult the *MOOteach mailing list on DU.
At TECFAMOO we made enthusiastic use of the Tutorial Room and a Generic Classroom which are put up on display in our MOOseum.
A very useful tool not just in virtual class- and tutoring rooms are the www-slates (by shkoo@eon) which allow accessing www pages from within the MOO. Those slates can be ``pretuned'' (or book-marked) to a set of pages, so that teachers can put on display somewhat preconfigured www-boxes in certain areas of the MOO.
Note that virtual classrooms and similar artifacts are not *the* only way to use a MOO. After all they just replicate more or less traditional teaching via distance. See section 5.2 for a start on other directions.