1. Virtual office hours. We used Moodle Chat, but I would use the analogous tool on Blackboard, which is the LMS of choice where I teach.
2. Guest Speakers. Skype would be a tool that can be used to bring a guest speaker into an online classroom.
3. Collaboration: I encourage the use Blackboard Collaborate for any synchronous group work. The current downside is the need for the instructor reserve time due to limited licenses. Google Hangouts could be an alternative in this case.