It isn't software, so much as it's access to people who work for a cell carrier, and have access to the database where IMEIs can be whitelisted for unlocking... and doesn't mind earning a little extra cash for breaking company policies.
There used to be lots of people with these resources, particularly for AT&T, but there have been crackdowns recently on who has access, and a s result it's been a lot harder to get an IMEI unlock without getting the carrier to do it directly.