The Christmas Episodes for season 1 and 2 contain part of the story 
and should be watched at the end of the respective season.

Planet of the Dead and The Waters of Mars are movies that should be watched after season 4 in that order.

Space/Time is a short about season 6 and Rewind is basically a review of season 5.

I do not own any of these videos, I found the torrents for season 1-5, season 6 and season 7 and 
the specials and compiled them into a single torrent for ease of access.