I think everyone knew that BBC America has started putting new(ish) Doctor Who episodes on iTunes, but I just discovered that they have a limited selection of Third Doctor episodes.
It's a little confusing, but each story arc is listed as a "season" and each episode is a separate download. It still ends up cheaper than the BBC DVDs, but paying $8 for 100 minutes kinda hurts (no special features or commentaries). I've bought a few 1 hour and 2 hour shows off iTunes for $1.99 in the past and I think they'd be a lot more successful if they halved the "season" price.
Anywho, here's a list of stories currently available.
Carnival of Monsters
Spearhead from space
The Three Doctors
The Time Warrior