Here's a semi-exhaustive list of a lot of the research tools I use. I never want to gatekeep information or research methods I use, so if y'all ever have issues trying to find something, just let me know. The more people researching, the better in my opinion.

I have a few methods that aren't easy to explain, but these are very straightforward:

Reverse Image Search:
https://tineye.com/

Mind Mapping Tool With Degrees of Connectivity:
https://graphcommons.com/

PDF Search Engine:
http://pdfyes.com/

Powerpoint Search Engine:
http://vadlo.com/Powerpoint_Search_Engine.html

Facebook Ad Library:
Useful for tracking political ads and NGO propaganda.
https://www.facebook.com/ads/library/?active_status=all&ad_type=all&country=US

OpenCorporates:
Database of most every company in the world.
https://opencorporates.com/

IRS Tax Exempt Organization Search:
Best way to find form 990-PFs, which lay out nonprofit financials.
https://www.irs.gov/charities-non-profits/tax-exempt-organization-search

FollowtheMoney:
Political spending tracker.
https://www.followthemoney.org/

Health Data:
https://healthdata.gov/

Pharmaceutical Side Effect Search:
https://www.druginformer.com/search/

Snapchat OSINT Multi-Viewer:
https://www.osintcombine.com/snapchat-multi-viewer

Nonprofit Foundation Information Guides:
https://www.foundationguide.org/foundations-trust/

Article Citation Index:
Search for authors, topics, etc., of scientific articles.
http://citeseerx.ist.psu.edu/index

Internet Archiver:
View old versions of sites or search through the index of millions of documents, photos, videos, etc.
https://archive.org/

Username Search:
Do a single search to see what platforms a username appears on.
https://checkusernames.com/

AI Search Engine:
A very powerful search engine that uses AI to find exactly what you're looking for. It takes a little getting used to, but it does an amazing job.
https://www.iseek.com/#/web

Directory of Open Access Journals and Articles:
https://www.doaj.org/

World Legal Information Institute:
Laws, legislation, treaties, law journals, etc.
http://www.worldlii.org/

CrimeSpider:
It's a now-defunct website with endless resources for investigating crimes. However, it was archived extensively and can still be explored very thoroughly via Archive dot org.
https://web.archive.org/web/20181011232223/http://crimespider.com:80/

UN Map Library:
Maps of pretty much anywhere. I know it's a UN site, but it's handy.
https://research.un.org/en/maps/

Old Maps:
A directory of old world maps.
http://www.oldmapsonline.org/

ReliefWeb:
NGO aggregate.
https://reliefweb.int/

Breaking News List:
Just a list of headlines from around the world in chronological order. No frills, useful.
https://www.1stheadlines.com/

Other Internet Tools and Resources for OSINT:
This site has a pretty extensive list of different resources and methods for internet deep dives. Very handy to have bookmarked!
http://www.onstrat.com/osint/