How to take screenshots of sections of the screen?

On Mac OS X, there is a handy key combo for taking a screenshot of any arbitrary rectangle section of the screen(Command-Shift-4).

Is there something similar(maybe Compiz plugin) for Ubuntu/Gnome?

Press shift + printScr and then a click drag to select the area you want a screen shot of

It is possible that the key combination shift+PrntScreen may not be the short cut to do this, then -

1. Open System Settings (also known as gnome-control-center)
2. Go to Keyboard
3. Click the shortcuts tab
4. Click on screen shots
5. Click on the key combination corresponding to "take a screenshot of an area"
6. Press the desired keycombination ( such as shift + printScreen )
7. use the key combination, your cursor arrow turns to a '+' like sign.
8. click and drag to select the area you want a screen shot of.

Command line interface way( i.e through terminal)(you need dconf-editor for this) -

1. open terminal using ctrl+alt+T
2. enter gsettings get org.gnome.settings-daemon.plugins.media-keys area-screenshot to get the key combination currently assigned to get a screen shot of an area.
3. If you want to change it entergsettings set org.gnome.settings-daemon.plugins.media-keys area-screenshot "[<Shift>Print']" Simly replace "[<Shift>Print']" with your desired key combination.

This is a link to ubuntu wiki about keybindings and is handy

Note: I have tried this method on Ubuntu 13.04, and I remember using it on Ubuntu 12.10. I know very little about the possibility of using these ways in Ubuntu versions before that..

Shutter is a good screenshot tool that has this feature, among others.

If you use GIMP for image editing, File - Acquire - Screenshot - Select a region to grab will do the trick.

Have you tried compiz screenshot option!
Enable screenshot in your ccsm and then use Super Key (the Windows Key) + drag to get a screenshot of whatever area on desktop you like. The shots are saved on desktop (/home//Desktop) or wherever you choose when you enable the Compiz tool.

Alt+PrtScrn grabs just the active window. Not exactly the box you requested, but useful if you only want to grab the app you are using.