I have been running https://pageproofer.com for 3 years. It allows web designers and developers to easily leave feedback and track issues directly on websites (like digital sticky notes). It has been profitable from year 1 and continues to grow month over month.

The growth is slow and steady but not at a point where I'm doing it full time. I don't think 'successful' needs to be determined as 'majority of owners income'. For me it's a lucrative side project that requires little attention day to day. It doesn't need to provide the majority of my income since it doesn't take the majority of my time.