I found a website (C-DAC) in India that performs prior art searches for free. But, you have to live in India to register and request for a search. Are there any other prior art search services for free?
Here are a few key resources: Patexia Patent Database
The United States Patent and Trademark Office (USPTO):
World Intellectual Property Organization (WIPO):
European Patent Office (EPO): EPO offers information about patents in Europe, including Espacenet, a searchable database of more than 70 million patent documents worldwide, from 1836 to today.