I drive around my neighborhood looking for scrap metal and stuff to use/recycle/donate/sell. This past Sunday I found a Scientific Atlanta Explorer 8300SD DVR with a 80Gb HDD and a Scientific Atlanta Explorer 3250 cable boxes. I have Brighthouse cable with their HD box. The 2 boxes I found 3 streets away from me are for Brighthouse too. The DVR says something like "This Explorer is not aoutherized for for use" when it boots up and won't do anything. Is there a mod/hack(legal or not legal) so I can use it or should I just scrap it and keep the HDD?

legal? no.

this would be the wrong website for "illegal" means.

The boxes won't work for you, legal or illegal. Original renters most likely will be charged for lost equipment. As a goodwill gesture you may want to just call Brighthouse, tell them what you found & have them pick them up (80gb not worth it imo & they may also give you some credit on service.)