Actually, they sent quite a bunch already on Mars. But what's special with Curiosity is that it's packed with far more experimenting tools and sampling processes. It weighs one ton, while the others were only a few dozens of kilograms heavy. Digging up a list of missions on Mars as of now ASAP.
Okay, there were six landers on Mars, the two Vikings from 1976 and Mars Pathfinder from 1997, and then Spirit and Opportunity in 2003 until 2010. Only the two last ones were mobile. And now, very recently, Curiosity.
Also, I ignore why the images provided by the NASA were of so poor quality and in black in white, but I know for a fact at least Spirit and Opportunity had high quality cameras, like Curiosity surely does. Here's a beautiful image from Spirit.
http://en.wikipedia.org/wiki/File:Spirit_rover_tracks.jpg