Google Fonts
fonts.google.com
A free library of over 1,500 open-source font families from Google, browsable and downloadable for use in any project with no licensing fees.
Why it is useful
The largest free, legally clear font library available — every font can be used commercially without attribution or fees. The pairing suggestions and preview tools make choosing complementary typefaces significantly easier for non-designers. Direct CDN embedding makes web implementation a single line of HTML.