What directory comparison tools can I use on OS X?

I am looking for a tool that is able to compare directories, not only files. Also it is important to be able to call the tool from the command line.

It would be great to have a free tool, if not free please specify the price.

For all the googlers... check out Beyond Compare it rules. Costs 30$ or 50$ for Pro version.

FileMerge (free), shipped with Xcode, offers a directory view.
A command line version is available through the Terminal application opendiff.

Here's how you can compare two directories with FileMerge:

1. ⌘+space, type in "FileMerge" and open it.
2. Click the "left" button and choose the folder you would like to move items FROM. (The "old" folder)
3. Click the "right" button and choose the folder you would like to move items TO. ("new" folder) and click "Compare" button
4. In the right panel, choose to exclude: "identical" and "Changed right". This way you will only see files which are missing in the "new" folder and ignore files your may have added in the "new" folder.
5. Move files manually in Finder or let FileMerge do it, by choosing an option in the "Merge" dropdown in the right panel.