The caption on the image from the National Geographic Site reads:
"The first attempts to reach Mars (1960) and Venus (1961) failed, yet triumph followed quickly. Of the nearly 200 solar, lunar, and interplanetary missions depicted on this map, most have been Earth's closest neighbours. As rocketry, navigation, and imaging have become ever more capable and reliable, the planets and many of their moons have become examined in detail. The New Horizons mission to Pluto is under way, as is the MESSENGER mission to Mercury. Others not yet launched, perhaps not yet dreamed, await."
Be sure to click on the image to see a larger version. If that still isn't big enough - you can see the full-resolution version here on flickr.