Research Papers

BioLine - Not a search engine but a collection of academic journals released for public use.

RefSeek - Search engine focused on indexing research sources for a variety of topics.

Science.gov - Scientific research released by the US government for public use.

WorldCat.org - Online library catalog to find resources in libraries near you.

Information Validation

QPublic - Nice little tool to see more information about a property. Specifically useful for validating the real address of a location. There is a delay and it is not a true mirror of tax record information, but is still incredibly accurate a great deal of the time.

USPS Zip Lookup - Incredibly useful tool to validate addresses. Inputting an address will result in seeing the full zipcode and may suggest corrections.

Miscellaneous

Shodan.io - A means of searching through internet connected devices. Alternatively you can view information it has on an individual domain or IP address and what records it has associated with them. It also has a fantastic API.