There are a variety of things you could add to your sidebar. If I had my own forum with a sidebar, these are some things I would add to it.
- RSS Feed
- Latest Posts/Threads
- Image Links to Social Networking Sites (Facebook/Twitter)
- Quick Search Bar
- General Links (Board Rules/Announcements/etc)
- Maybe a HTML/Java powered Clock that shows time custom for each users timezone
There are so many more things you can do. Just be creative!