I find the big unregistered banner offensive and distracting myself. How about..

- you could also run it as a service for a monthly fee, hopefully marketed by isps to their customers.

- or offer registered users a private mailing list with new plugins at discounted prices.

- or crippleware, but provide screenshots so they can see exactly what they'll be getting.

- or half way through, you let them enter sample products into an auction on your site (or you provide a demo site). if they bite, let them download a public key encrypted version of your source code. once they pay you, you send them their unique private key. giving them source code is a positive thing, and they need it.

- conceivably you could search for it on google and tell the difference between sources with md5 or statistical analyses of white spaces or perhaps embed info tags which don't show up like the width in pixels of a transparent gif, or the hex digits of font color tags. maybe encode a key in the background colors of cells in a table? :)

- seriously you could probably embed a liscense number in a single color tag, that's easy to interrogate and will cover way more users than you will have.

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