I don't know any good open source software for this, but if that is not a hard requirement, I have used ABBYY FineReader 9.0 Express Edition for a couple of years now, and I am generally pleased with the quality of the OCR results. Not sure if I can say better than 98% but good enough to be easily correctable.
I use it mostly to OCR Japanese text from books or magazines. According to its recommendation, and my trial and error, very high resolution actually gives worse results than sticking to 300dpi.
The only consistant issues I came across:
1) Sometimes it will return a Chinese kanji that is similar to the actual Japanese kanji. I don't know if this can be restricted or just because the OCR supports both languages.
2) Some character might be incorrectly interpreted through an entire document. But after the first ones it is easy to Find & Replace.
3) It doesn't seem to be able to read English when mixed on a page. I just enter that when proofing.
It looks to be up to v12 now. Possibly you could find an previous version for a good price.