For me it was Numega's SoftICE, which was an amazing debugger. Using it to defeat it's own protection always struck me as ironic, but I guess also to be expected.

I hacked software for many years, for my personal amusement rather than profit, as it always reminded me of being a teenager hacking games for infinite lives, etc.

Now and again I take a stab at commercial linux software, but there isn't so much out there with the standard "Enter License Key" kinda protection.