As a project based staff, this happens to me always (when I've project, I'm damn busy, otherwise I'm idle)

I totally understand how you feel. I don't purposely act 'busy', but that doesn't mean I can do anything (e.g., watch movies, comics etc.)

Usually I'll first ask my colleagues if there's anything to help, if no-one requires my service, I'll just
- Do some housekeeping tasks, clean the junks inside my laptop
- Sort and organize my emails, contacts etc
- Read some articles in wikipedia