GIMP mostly. That is what I learned on so it is easiest for me. I have photoshop elements and have used it a few times, but I still find GIMP easier. I started with it in 2008 I think!
I also use Paint.NET! I love that program. So fantastic and free! Once I figured out how to install the plugins and I could open a PSD file, my life changed!
I actually started with Photoshop 7, the last version before it became CS. I cried, literally, trying to teach myself and could find no resources or people to help me. So I bought a little Wacom that came with PSE4, the only version for Mac. I've gone through almost every other version of PSE and could kick myself for not buying CS6 which was the last one where you could actually 'own' the program. (I know Adobe never uses that term but too bad!) I really have it as a goal to jump into PhotoshopCC because I already pay each month, mainly for my LR but may as well bite the bullet and use the other half of my $.
This was Cheryl and me! We finally did it because we were already using Lightroom. You can do it! For me? I had to commit. I had to delete the PSE shortcut off my computer and leave the PSCC one. Just making it that much harder pushed me to open CC first.
I have always used photoshop ... I don't know anymore how many version I used .... every new version was an improvement ... more possibilities ... Now I use it together with LR.