whiteboard in Unity3d

Is there any easy way to write a script to handle a whiteboard, where the players can draw anything, and the others see it?

You can try using UnityPaint.

Also, check out http://forum.unity3d.com/viewtopic.php?t=57722&highlight=raycasting+paint+texture

Search the forums, if I remember correctly somewhere there's a project someone posted that does almost exactly what you need...