The rule I am going to adopt in listing them here is similar to that I've used for the individual posts on actual companions. In order to count, they have to be contemporary with the series at the time, rather than being added in new stories featuring old Doctors after the fact. In this case, since we're considering companions of the 8th Doctor, that means that they have to have appeared between 1996, when the TV movie aired, and 2005, when the new series did. It's arbitrary, but I have to draw the line somewhere, and that's where I've drawn it so far, so let's keep it that way.
Leaving the comics to one side for the moment, the novels re-launched in 1997, now under the auspices of BBC Worldwide. Unable to use Grace Holloway for legal reasons, they had to invent a new companion. Apparently deciding that since the people who watched Doctor Who in the '70s and '80s had largely been teenagers, and they surely weren't likely to have aged since (right?), they settled on Generic Teenage Girl as their model for the companion. So, first appearing in the universally derided 1997 novel The Eight Doctors, we have Samantha Jones, better known as "Sam".