I have ideas for a number of desktop applets that I'd like to develop but I can't find any information/resources to tell me how to get started. I use/have used Ubuntu-based distros but it's not clear to me whether I have to pick a desktop environment like Cinnamon or gnome and write exclusively for that, or whether I can develop more broadly and have it work in multiple DE's. If I was to pick a single one, I'd probably pick Cinnamon or gnome 3, but I can't find any documentation for either of those that tells me how to develop desktop applets for them. Is there a secret handshake I need to know to gain entrance to this club?

Check out this link - this gives plenty of gnome 3 tutorials and examples.

https://developer.gnome.org/gnome-devel-demos/unstable/

Gnome and Cinnamon share alot and both use GTK+ so applications written for one should work fine with the other.

In fact as long as the target machine has the libs they should work on any desktop - but loading libs will take more resources.

Thanks for the response, archShade!