Originally Posted by martz: “How exactly do these work, i've seen ones like this http://www.dreamgsm.join.pl/sklep/images/D500.jpg and dont quite understand how something that small can unlock a phone”
They use something called a pic chip, which basically stores a small program which unlocks the phone.
If you have electronics experience, many of the older versions of the code (aka hex file) to unlock phones such as the Samsung A300, are now free. So if you have some breadboard and you can buy a pic programmer for about £20 and a few chips for £2 - £3 each and have a go yourself.