Atmel gave out the IDE for free, then someone noticed and came out with the Arduino. Bam! Instant market penetration.
Actually Wiring (http://wiring.org.co/) was first, then Arduino took the code to use it on cheaper chips. And Arduino keeps using code from Wiring, even today, without a proper attribution. It's true they stick to Wiring's license, but it would be nice if they let the world know it wasn't their idea.