There are no free email databases that can be legally accessed. However, there are companies that compile and sell large email lists consisting of the users that subscribe to websites to other companies. These lists are often sold to other companies that are looking to creating an email marketing campaign.
Companies, such as Database USA and Health Link Dimensions, organize and sell large quantities of mailing lists. These lists often comprise people whom all have a common hobby or interest, so companies can use marketing emails and tools to reach them. Health Link Dimensions, for example, creates lists with physicians from around the world. Each email package can be sculpted to adhere to the marketer's wants and include delivery times as well as number of emails to send.
There are some legalities to adhere to before either selling or purchasing email lists. The first thing to look for is to see whether or not the email participant is opt-in. Opt-in participants are those that chose to include their emails as part of a mailing list, and are completely legal to send messages to. Marketing companies will often deny the use of non-opt-in lists because the emails will be regarded as spam.