Is there a screen-scraper for Linux?
I am not exactly sure what you mean. However, if you are talking about screenshots or capturing what is on the screen, Ubuntu has a very good built-in program.
On the menu, expand
Applications > Accessories > Take Screenshot to launch it.
if you need to "take a photo" of your screen or a window, then there should be the built in app as William mentioned - in GNOME desktop environment.
If you use KDE, then there is Ksnapshot. In both cases, you can try to press PrintScreen (prt sc or PrtScr) on your keyboard and the app should start.
If you need to extract text from image, then look for OCR apps like GOCR etc.