gocr works great and it's simple, just invoke it like so:

gocr filename.png

The output will be printed to the screen.

My only complaint/concern is that even with standard terminal output (not scanned) from a printscreen, gocr does make mistakes by inserting extra spaces where they don't belong, mistaking letters for numbers etc.. but it's definitely enough to be readable and figure out what you're looking at.

I haven't tested yet with scanned input from invoices, receipts etc..