reCaptcha was always a free captcha service based on one simple idea: one part is the classical one, hopefully only readable by humans, the other one actually supports Google on doing OCR-like jobs. Previously it were undetected words from Google Books scans, now they are mostly using house numbers.

More interesting is the other change: reCaptcha now tries to detects real users and then only generates a simple number captcha for the classical part. Likely they are using your Google Account cookie or Google Analytics for that.