ZCorp and Objet are leading the way in making their presence known globally. They regularly publish their own blogs and comment far and wide on forums and social media sites. They are getting significant profiling as a result of their efforts. There is also a notable but more subdued presence from Stratasys, RepRap (& derivatives), Mcor and 3D Systems (although DDD gets a lot of coverage from many different sources due to its acquisition strategy - I am a good example!!).
They probably already know who they are, but just in case — EOS, Arcam, Optomec, Fcubic, Envisiontec, Renishaw (previously MTT), Huntsman, Concept Laser, Voxeljet and Sintermask. These are some great companies doing some incredible things with additive technologies — but you need to share more :-)