I spent many afternoons trying to get tesseract to read Dwarf Fortress screenshots, such as http://i.imgur.com/32vVhnH.png - including much pre-processing, such as converting the text to black and white. Alas, I never even got close.
Edit: just tried Google's, and it had one mistake for that entire file. That's pretty impressive.
I spent a weekend tying Tesseract together with Tekkotsu (the amazing open framework for the Sony AIBO) in an attempt to teach my robot dog to read. The eventual goal being to hook up the output of OCR --> Text To Speech (TTS) and have him read to me.
Alas, the low resolution of the camera was an insurmountable problem. Poor Aibo needed 40-point fonts and I practically had to rub his nose in the book. Not exactly the user experience I was aiming for.
If you're reading screenshots of a non-changing font, you could quite easily get away with plain template-matching. Simply do a run through and label the data one time.
I did something like that a few years ago when making an Eve-Online UI scraper.
Edit: just tried Google's, and it had one mistake for that entire file. That's pretty impressive.