Free tools:

European Patent Office (Espacenet) - Some patents are available in full-text, while others have only abstracts or bibliographic data. Covers about 90 patent authorities. It is recommended to combine the advanced search with the classification search.

Google patent search - Google Patent Search covers the entire collection of patents made available by the USPTO - from patents issued in the 1790s. The advanced search is available in this link: https://patents.google.com/advanced
This database covers 17 patent authorities. It creates graphs and timelines showing the leading inventors / assignees.

Israel Patent Office - Patent databases in Israel and abroad.

The Lens - The Lens is an Australian database covering 95 patent authorities. It provides search for more than 30 million scholarly articles, across various disciplines and from multiple sources including those cited in the patent literature. Biological sequences are also searchable. It is recommended to use the structured search and the scholar search. Signing in enables to create personal folders and save data. 

USPTO - U.S. Patent and Trademark Office: Full-text since 1976, full-page images since 1790.

WIPO - World Intellectual Property Organization (Patentscope) - The World Intellectual Property Organization (WIPO) is a specialized agency of the United Nations. Its headquarters are in Geneva, Switzerland. It covers 41 patent authorities. It is recommended to use the feature "Field combination". The database enables to search also by nationality or applicant address country. In the results page there is an "Analysis" button enabling further processing of the information. The documents are in full-text.