I’m not sure how I missed this. A while ago I posted an article from DIYDrones about a guy who was working on an optic flow sensor based on a cheap mouse sensor. It looks like the fine folks over at DIY Drones productized it and now sell it at their store. Awesome! There are some setup instructions available also. Developers also might find these papers on obstacle avoidance and position hold. As they outline in the instructions this sensor has a few limitations but I think some could be overcome with some clever sensor fusion techniques and the application of multiple optic and IMU sensors.