Open source OCR tool available in the market

Use Tessnet.

Tessnet is C++/CLI .NET Wrapper for tessdll (and tesseract) for .NET 2.0.

For what it is worth, there is also GOCR. Not tried.

There is Tesseract and although it has no native .net bindings, it's pretty complete.

The "one to watch" is OCRopus - A Google-sponsored effort that isn't ready for production work yet.