Support Across Lite puzzles (.puz)
These are (at least were) very common, and there's a giant corpus of them from newspapers et al.
There's the text and binary formats. The text format is used by authoring tools it seems, and the binary format is what published puzzles use.
Attached is the text format documentation. I don't know if the binary format is documented.
Free software implementations include the (unmaintained?) libpuz, used by a long since dead previous Gnome crossword app.