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Why not implement the keygen using public key cryptography? Wouldn't that eliminate the possibility of a reverse-engineered keygen right off?

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If a "license file" is acceptable, this works great -- you can use a human-readable text file describing the license (expiration, features, etc.) and sign it inline with PGP. I've seen Citrix XenServer use this approach, for example. However, even just the signature is really too big to use as a serial number replacement. ECC might work.

If you sell the software digitally the size of the "serial number" isn't a problem.

Just put it on a web page (and in an email, for later use) and have the user copy-paste it. Or even better, do what patio11 is doing: when the user clicks "Buy software" in the app, add a magic number to the URL you send the user to and have the software poll the server to see if the user has paid yet.

Also: poll the clipboard to see if the user has copied a valid serial key, to save the user the hassle of pasting it in the correct input box.

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