On September 30, Apple asked developers to start sending their applications ready for OS X Yosemite, so that they will be available on the Mac App Store when the new system is released.
OS X 10.10 has everything to be released to the general public tomorrow, but some believe that Apple will be able to launch it next week, more precisely on 10/21 (the day after the company's financial results conference, as she did in the last two years).
However, the new version of PCalc (4.1, fully prepared for OS X Yosemite) has just landed on the Mac App Store, which indicates that the system release has everything to happen tomorrow!
In addition, it is interesting to see how a “simple” calculator can take advantage of the new features in OS X 10.10. Now the app has a widget for the Notification Center, supports Handoff (start doing a calculation in OS X and end in a iGadget and vice versa), import of custom layouts (with synchronization via iCloud), dark theme (to match Yosemite's new dark mode), among other things.
I hope that many developers will follow the example of PCalc and integrate their apps with the new system in this way.