These tend to fall into two different categories; apps that crowdsource data collection, and apps that allow for mobile data collection.
Citizen Science is perhaps the best example of crowdsourced data collection. There are an amazing number of apps devoted to this; from conservation-focused initiatives such as MammalTracker and PlantTracker, to astrophysicists using Crayfis to turn smartphones to detect ultra-high energy cosmic rays!
Mobile devices are also changing the way we collect data in the field. There are a plethora of apps devoted to paperless forms, as well as data collection with embedded geolocation.