Over on his blog Daniel Estevez has created a post showing how an RTL-SDR can be used to receive, plot and forward RS41-SGP radiosonde data to the APRS-IS network. Radiosondes are the small payloads used on weather balloons. They transmit weather and positional telemetry data back to a base station at the meteorological agency. But depending on the frequency used in your country it can be fairly easy to receive this data yourself with an RTL-SDR dongle and some decoding software. We have an introductory tutorial for radiosonde decoding available here.
In his area of Barajas, Spain the meteorological agency recently switched to the newer RS41-SGP radiosondes. To decode these Daniel uses the open source "RS" software which is capable of decoding various radiosondes including RS41. He notes that for now it is better to use his fork of "RS" as the base version contains a bug. He also shows how the received data can be plotted in Viking, which is a program used for plotting things like GPS tracks on a map.
Finally he shows how to feed the radiosonde data to the APRS-IS network. APRS is a packet radio system used by hams which works via radio and the internet, allowing for worldwide communication by radio. Feeding the data into APRS-IS allows anyone to see the flightpath on a site like aprs.fi.