SpotifyInstant and CollabLists are two applications that extend the features of Spotify for users. SpotifyInstant allows users to search for songs on Spotify and instantly start playing them, without having to navigate through the app. CollabLists, which won 
3rd Place at the 
Facebook Regional Hackathon in 2014, allows users to create collaborative playlists with their friends. These apps enhance the user experience on Spotify and make it easier for users to find and share their favorite music.
CollabLists
CollabLists is a music app that focuses on solving the problem of bad music at parties. It allows users to add, vote on, and delete music from a shared playlist using Socket.io, an open-source library that enables real-time bidirectional communication. The app has a frontend written in Handlebars.js, a simple templating language, and a backend built with ExpressJS, a web server framework for Node.js.
The app integrates with the Spotify desktop application using the Spotify API, which allows it to access and manipulate the user's Spotify library and playlists. This allows users to add songs from their own personal collections to the shared playlist, as well as see and vote on songs added by others. Socket.io is used to instantly sync the playlist between all clients that are connected to the service, ensuring that everyone at the party has access to the same music.
CollabLists was built in less than 24 hours for the Facebook Regional Hackathon and won 3rd Place. The app's use of real-time communication, integration with the Spotify API, and focus on solving a common problem make it a unique and useful tool for partygoers.
SpotifyInstant
SpotifyInstant is an application I built that uses the Spotify API and jQuery to enable instant search for Spotify.
SpotifyInstant application. See note below for details [link]
When the play button is pressed, the search results can be heard in the user's desktop app. It's important to note that this application requires the Spotify desktop app to be installed. This project is a great example of utilizing popular APIs to create a useful tool for users.
Note: Spotify has updated their APIs and authentication flow, and these applications no longer work.