Ok so it took me a couple minutes to understand what this is. Do you have examples of how this is used, what kind of hardware provides networked sensor data? Is there a compatibility list somewhere?
The main hardware that others have used are Arduino, Netduino, and ioBridge. But, you can do more abstract things, like use CURL to send server uptime or embed a web page counter on your website that sends a Growl update or flicker an LED.
Here are some tutorials - http://community.thingspeak.com/tutorials/
Here is a project from me using netduino and jQuery to build a front-end showing updates from my house.
http://iamshadowlord.com/2011/02/thingspeak-building-my-own-...