Apple's Preview program in OS X is great for capturing signatures, but by default these can only be used with Preview; however, there is a workaround to extract and them for use elsewhere.
Data Export Options
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How-to: Use Preview to put signatures on PDF's, Pages Documents
How to extract Preview's signatures for use in other applications
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How-to: Use Preview to put signatures on PDF's, Pages Documents
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Preview Survey
How-to: Use Preview to put signatures on PDF's, Pages Documents
Evaluation version
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