If in the past you've relied on Jonathan Snook's cheat sheets for previous versions of Prototype, now there's no excuse not to upgrade. Kangax's version catalogs the full Prototype API, but marks in red those methods that have been deprecated in 1.6.
Friday afternoon isn't the best time to announce a new release, so many of you may have missed the unveiling of Prototype 188.8.131.52 several days ago. It's a bugfix and performance release that marks the beginning of our official support for Opera 9. Read the previous blog post for more details.