Does it have to be open-source? If free, but not trainable and restricted to Windows apps/phone is good enough, then I recommend the Microsoft OCR library. It gives you very, VERY good results out of the box. An excellent piece of work from Microsoft Research. To test it, see for example https://ocr.a9t9.com/ which uses Microsoft OCR inside.

And for comparison, an OCR application with Tesseract inside: It has a dramatically lower text recognition rate: http://blog.a9t9.com/p/free-ocr-windows.html

(Disclaimer: both links are my little open-source side projects)