A purpose-built beat battle platform integrated with SoundCloud, complete with profiles, voting system etc.
We got a couple thousand users in the end, including some notable artists from the LoFi scene (they're kind of a big deal... people know them).
Eventually we shut it down so I could work on bigger things (Curl), but it was a good first side project which ended up finding some surprising success.
"Attention to detail" isn't so much about pixel-perfect design as it's about recognising details that can have an outsized impact:
The record (made of the user's profile pic) spins at 33rpm when a track is playing, i.e. the standard speed of a real-life turntable.
The tone arm tracks across the record as the SoundCloud player progresses, and it resets in a life-like way when the track finishes.

This is the kind of detail that can create a disproportionate payoff in user loyalty and brand advocacy, because it communicates the kind of shared understanding that you can't really fake.
There's a lot I would change about the design if I did it again today, but pretty happy looking back on this. 😉