I have found that agency and video production is usually based around a sub-set of the larger team getting together for a duration of an event or project.
To facilitate the changing team members as well as shifting priorities in these environments a tool such as Trello works well:
You might be interested in trying out trello.com. We use the higher end paid version and have been really happy with it. It's very visual and customizable. I've seen it in use at an agency of 40+ employees as well as smaller 5-6 person production houses and it seams to do really well.
If you're looking for options:
If you're looking for a flat structure, try Asana. Looking for customized apps, try Podio. Security and conference calls, try Unison. Quality, visual structure, try Trello.
My first four to try in your situation would be Podio, Trello, Basecamp and Asana.