I agree with Charles Boyuing, however I will add that I personally follow the Microsoft Windows 7 UX Guide quite closely in order to bring consistency to my applications and have them fit in well with the rest of the Windows 7 platform. I know that in Gnome development there is the HIG and KDE has some documented guidelines but as for Apple I don't have anything better to offer you than what you already know.
Some of these guidelines are specific to a given platform or environment but a great deal of them overlap and it's in those areas I think it is good to make sure any well written app tries to follow these common conventions.
Since my original answer there have been some changes in different ecosystems that I'm aware of that are worth mentioning.
Google seems to be centering lately around the Material Design guidance with many of their web and mobile applications. With the increasing number of popular desktop applications using Electron this may be a common design language to keep an eye on.
Microsoft as of late has been pushing forward with Fluent Design System as a way of move forward from the previous Metro/Modern design guidelines. Fluent Design seems to be less limiting in the types of UXs created compared to the previous Windows 8 guidance and has been used in developing new parts of the Windows 10 user interface.