Online sensor data feeds, so you can experiment with other people's sensor data outputs: 

• Knoesis meteorological datasets (downloadable data) 

• datahub.io sensor datasets (downloadable data)

• Safecast data API (api)

• Open Weather Map (api)

• New Zealand climate data stations

• Air quality in Scotland http://datahub.io/dataset/air_quality_in_scotland http://www.scottishairquality.co.uk/ 

Other places to look for sensor data: 

• Sensor Data Storage Websites - wikipage
• https://community.spark.io/