The Five(ish) Doctors Reboot is a Doctor Who 50th anniversary thing some of the Classic Who series actors did in co-ordination with the rest of the 50th anniversary stuff.
It's a little over a half-hour long and written and directed by Peter Davison, who played the Fifth Doctor. Hilarious and touching, with some mind-blowing cameos from unexpected places. There is stuff in the credits too so stick around.
It's currently available internationally for free through the BBC
here, or through BBC iPlayer for those with UK IP addresses.
It's not (yet?) available for purchase through iTunes, though there are a whole lot of goodies available in addition to the 50th Anniversary episode and the Docudrama about the beginnings of the show that was scripted and produced by Mark Gatiss, and some little things may still be available for free there, so check if there is a section for BBC programming in your country's version of iTunes.