CS2 is the older version of Photoshop. CS3 is the current and newer version which includes a lot of extras that CS2 does not. Still, CS3 costs a hefty amount and CS2 is free if you get over there right now, so if you have been wanting Photoshop but been kept out of the market by the price tag, now is your chance.
Apparently Adobe couldn’t be bothered actively supporting valid product activations for CS2 anymore given that it’s about 10 years old, so they will let everyone download and use it for free. Hooray!
UPDATE: Apparently Adobe have acknowledged that making the links and serial numbers available to everyone, and not just for people who had a previous license for the product, was a mistake. Since it is too late to fix this mistake and take back the software from everyone who already downloaded it, they seem to be just leaving it be and allowing people to download and use the software without having a previous license.
Good idea on their part I say. This way they come out of it looking generous, and gain lots of extra users who (like me) couldn’t afford Photoshop but will now use it and possibly buy a newer version of it in future.