Screen capture is useful in MANY ways while collaborating. Two great programs for Macintosh are Skitch and SnapNDrag. Word of caution here. Just because you CAN capture anything that is on the screen, does not give you any right to re-publish copyrighted material! As a teacher, screen capture is great for creating illustrated tutorials. As a collaborator, let your partners see exactly what you mean.
I know there are some good (free) programs out there for Windows also, so if you have used a screen capture program and think it is a good one, recommend it here through the comments. Be sure to include information for where to find it!