OK, the title is a lie.
All things considered there are no alternatives to PS at all.
But for working in RGB there are several options.
I made my last ‘2D graphics apps test ‘ 2 years ago, mainly focused on RAW converting (I choosed and bought ACDSee Pro), but I found a pretty usable app, called Serif PhotoPlus.
It has similar GUI and features like PS has.
Serif offers a free version of the app with limited functionality (which is still pretty usable for retouching).
Unfortunately there is no trial version of the full app, but I had a chance to take a look on it and as far as I know, this is the only 2D app on Win which has comparable layer functions and operations to PS.
Of course it was slower than PS, but still was more usable than Corel Paint Shop Pro.
It only costs 85 EUR, which is quite acceptable.
By the way, the lack of the full app trial and the stupid way of downloading the free app (you have to register, etc.) deserved to put Serif (as a developer) to the ironic ‘How to develop an app’ posts soon.
On Mac it worth to give a try to Pixelmator (I will check it when I will have access to Macs again; I heard good things about this one), and soon I will try PS Elements again (I have an old one; I’m curious what has changed).
Maybe you are surprised that I don’t consider GIMP as a choice; well, although I use it a lot, I don’t like it at all.
It needs recoding and GUI change, too (in my opinion).