https://www.musictaco.co.uk - Find the cheapest place to buy albums online (you can also track your favourite artists and it will email you when they have a new release)
To be fair, I built it primarily as a way to teach myself Rails, and in this regard it was a great success. But in the back of my mind I figured I might be able to make some affiliate revenue out of the traffic but it never took off.
It has failed so far because: it is a small niche (most people just use Spotify these days), I haven't marketed it very well and haven't found the niches where likeminded people hang out (building is definitely easier than marketing). I could definitely do more on the homepage to explain the features, particularly around tracking artists and being notified when they release new albums, but at the end of the day I just haven't been able to market it (nor did I spend a lot of time on this aspect of it). For a long time it only worked in the UK (an even smaller audience compared to the global market), although I did just roll out an update for it to work in the US, but have not done anything to advertise the fact.
Feedback welcome!