I thought about this as well. I went and registered instacaptcha.com but I never managed to do it and the domain expired.

Seems to be doable. The user pays 1 usd/month and gets 100 credits. The extension author can outsource the solving to http://antigate.com/ and get the answer in 15 seconds.

There's webvisum and Captcha Monster. Captcha Monster is like $5 per 1000 captchas or something. Not sure about webvisum.

If someone could use [something like this](https://github.com/mekarpeles/captcha-decoder) to make an extension it would be great.