Recently, Adobe sort of messed up.  To support legacy customers, after turning off their activation server, they offered full version downloads and non-activation-requiring serial numbers on their web site.

This however means anybody with an internet connection can download and install several old Adobe products (eg. Photoshop Suite CS2, Acrobat 8 Pro, etc) for free.  All the information is available here: http://slickdeals.net/permadea.....mac-digital-download

Note though that Adobe has come out and said these files and keys are ONLY for paid customers.  Whether or not that legally entitles you to access them is up for debate (since their own EULA says downloading the software from Adobe entitles you to a license).  So for the moment, the biggest hurdle is your own conscience and whether or not you want to run 7+ year old software.

If, like me, you'd prefer a free photoshop alternative without any legal ambiguity, Lifehacker has recently posted an article of 10 excellent options:
http://gizmodo.com/5974500/10-photoshop-alternatives-that-are-totally-free