DronePlayer is a fun, interactive practice assistant for improvising musicians. Playing over extended tonalities is a great way to develop your ears as well as your musical ideas.
Long, sustained tones are generally known as drones. Drones have proven themselves quite useful for practicing. Many people use them to assist with intonation. Some enjoy playing scales over them. I personally have found them useful for practicing improvisation and improving my ears.
Muting one of the drone voices is a great way to play with just a fundamental pitch. You can then improvise over this and focus on hearing how what you are playing relates. Adding a second drone voice just adds to the fun.
Rotating DronePlayer to landscape orientation invokes a Drone Sequence mode where you can play evolving tonalities. These are excellent for challenging your improvising skills and having fun while doing so!