This blog post contains a currently not-working link that is reputed to be Adobe offering Photoshop CS2 for free:

Have an Older Computer? Download Adobe CS2 Premium Plus for Free | Fstoppers

I suspect the Adobe site is being slashdotted right now, so I wasn't able to verify the offer.

For reference, CS2 has most of the features used in tutorials on this site. There are some interface differences, no smart filters, less support for 32-bit images, and I think no 64-bit version.