Citation generators let you copy/paste a URL, ISBN, and DOI number to create a citation in various styles. They also let you manually-enter author, title, publisher, etc. to create citations. These tools are not always correct. Always check your citations against the actual style guidelines required and check with your instructor about how strict they will be about the formatting (spacing, capitalizations, italicization, etc.)
ZoteroBib - build a bibliography instantly from any computer or device, without creating an account or installing any software.
Citation Builder - free citation generator from North Carolina State University Libraries.
MyBib.com - Allows manual-entry and automatic generation of citations.