When planning to attend South by Southwest Music Festival, I couldn't find an easy way to see which artists I listen to would be performing. I'm always adding songs to my Spotify library even if I don't know the artists themselves. Plus, most concert aggregators out there would list your favorite artist in a five-hour concert of many artists.
Your Spotify at SXSW was a web app that scrapped SXSW's official schedule because it was constantly fluctuating. A user then could then login to their Spotify account so the SXSW schedule could be compared to their song "likes".
The app ended up being extremely popular within it's first year and I received lots of requests to update it for the next year too. Personally, it helped me see artists where I knew I liked one of their songs and ended up liking many more.