1. Big ones to scribble out design ideas
Whenever our team has a meeting we break out the big whiteboard and pens and start sketching out our thoughts. We don't use any formal notation (UML on a whiteboard?!), just intuitive scribbles that help people less familiar with the work rapidly grasp what we're talking about.
2. Medium ones to keep track of tasks
Each team has a whiteboard at the end of their group of desks (pod). This is used to stick user story cards on a grid to show progress (sort of Scrum-Ban or Kan-Rum). As other team members stroll on by they can see what we're working on and often suggest commonality that can be brought in from other projects. Several times we've been stopped from re-implementing code that has already been written for something else but not widely disseminated.
Having the white boards available was an immediate hit with the teams and has definitely helped us to communicate more effectively with each other.
3. A single big one with the overall server map
We run a fairly large collection of clustered servers covering many application tiers, and having a whiteboard dedicated to showing the server and application interlinks is very handy, especially as it's near the water cooler. It gets regularly updated with changes to application deployments (version numbers etc) and helps us all to keep in touch in a way that wouldn't be as effective through a wiki or email.
Sorry no picture available.