As far as I understand, every DVD player is originally manufactured region-free. It is then adjusted to its region of sale, or left region-free and sold as such. The adjustment is often made using simple codes through the remote, and people have found out these codes, and can use them themselves, these are the most common kinds of region-hack. I have successfully hacked 2 out of 2 players using these hacks and made them from region 2 to multi-region. In fact when buying the second player, we were deciding out of three players, and I checked the internet to see which one we could hack, and bought it for that one reason, as near half of my DVDs are region 1.
'DVD Region Hacks' has most of the well known hacks.
There are also some players that can't be hacked this simply, and there are companies who offer to make them region-free by adding chips and adjusting the actual hardware. I don't know too much about all that and personally it's not the kind of thing I'd have done.