tl;dr Sonic 2, some DLC, plus a few spinoff/prototype games. At the time of this posting only Sonic 2 and the DLC are free, but the other stuff should start becoming available one per day or so.
Free to keep but expires October 19.
You can get the steam version of Nights by registering your account at https://www.sega60th.com/
Note that the NiGHTs promo is while codes last, and when prompted to link a platform you have to pick Steam. Doesn't seem to be a date cutoff.
There's also "weekly content drops" available on the site, starting on October 21. Unclear what the content will be.