Hadoop is an open source Apache project, but a lot of the contributions come from Cloudera.
The Cloudera Distribution of Hadoop (CDH) appears to be the defacto standard, although other vendors such as IBM have their own. Cloudera provides a downloadable VM with a fully configured single node of Hadoop. I was able to get this up an running on my own MacBook Pro running Oracle Virtual Box in about 15 minutes.
Cloudera claims that they have more customers and more experience thatn any other Hadoop vendor.