Files with no extension
Classic Mac OS didn't need extensions: it kept each file's type separately. Copied to a modern computer, those files often look like nameless blank icons. Drop them on this page and it works out what they are from what's inside.
Which programs
MacWrite, MacWrite II and MacWrite Pro; WriteNow; Microsoft Word for Mac 1 to 5.1; Microsoft Works for Mac; Nisus Writer; FullWrite; Mariner Write; GreatWorks and BeagleWorks; TeachText and SimpleText. These are read with libmwaw, the open-source library LibreOffice uses for old Mac files.
MacBinary, BinHex and resource forks
Some formats keep part of the document in the "resource fork", which is lost when a file passes through Windows or email. If you have .bin (MacBinary) or .hqx (BinHex) copies, drop those: they keep both parts.