It was CS2, and the reason was because they were taking the serial verification servers off-line, but they didn't want to cut off the existing CS2 users. As to why they left the download page exposed to the public, there was a lot of winking and nodding going on, but as you said, no official word about it. Officially, you're supposed to have a legitimate license for CS2 products in order to use the unlocked versions they made available.

Later editions of the software do have some killer features, such as smart objects/filters, which can finally make your Gaussian blur non-destructive. The 3d functions let you wrap your map around a sphere without having to leave PS, and even paint on the sphere, which can be handy for sanity-checking an equirectangular projection. And the content-aware fill is simply amazing for doing things like removing a road from a satellite image. Although most of the time the Patch tool can do that just as effectively, if not as quickly.

Anyway, yes, the Gimp can do probably 95% of the things Photoshop can do, and you'll honestly have to get pretty deep into it before you find something you just can't accomplish with it. And yes, it does have some features that Photoshop does not, such as customizable fractal noise and the image pipe. It's also got a much more responsive scripting community, which is a huge asset. Inkscape is not quite as good a replacement for Illustrator, but you'd still have to be a pretty advanced user before you'd notice the limitations.

I always say that if you have a small budget, you should spend it on a tablet and use free software.