David Tennant Tennant played Barty Crouch Jr. in Harry Potter and the Goblet of Fire and the Tenth Doctor.

Michael Gambon Gambon played Albus Dumbledore in the Harry Potter films, and both Kazran & Elliot Sardick in the Doctor Who episode A Christmas Carol.

Helen McCrory McCrory played Narcissa Malfoy in the Harry Potter films, and a Saturnynian queen named Rosanna Calvierri in the Doctor Who epsiode The Vampires of Venice.

John Cleese Cleese played Nearly Headless Nick in the Harry Potter films and has a small cameo in a Doctor Who episode titled City of Death.

Zoë Wanamaker Wanamaker played Rolanda Hooch in the Harry Potter films, and the last living human named Lady Cassandra in several episodes of Doctor Who.

Jim Broadbent Broadbent played Horace Slughorn in the Harry Potter films, and played a possible future version of the eight doctor in the episode The Curse of Fatal Death.

Toby Jones Jones voiced the character of Dobby the House elf in the Harry Potter films, and played the Dream Lord in the Doctor Who episode Amy's Choice.

Bill Nighy Nighy played Rufus Scrimgeour in the Harry Potter films, and has a cameo as Vincent van Gogh expert in the Doctor Who episode Vincent and the Doctor (which will make you cry if you ever watch it).

Roger Lloyd Pack Pack played Barty Crouch Sr. in the Harry Potter films, and played the creator of the Cybermen, John Lumic, in Doctor Who.

Shirley Henderson Henderson played Moaning Myrtle in the Harry Potter films, and played Ursula Blake in the Doctor Who episode Love & Monsters, where she had her face fused with a slab of pavement by The Doctor.

Adrian Rawlins Rawlins played Harry Potter's father, James, in the films and played the leader of the Ood rebellion in the Doctor Who episode Planet of the Ood.

David Bradley Bradley played Argus Filch in the Harry Potter films, and voiced the vulture humanoids known as the Shansheeth in Doctor Who.

Derek Deadman Deadman played Tom the Innkeeper in Harry Potter and the Sorcerer's Stone (he was later replaced by Jim Tavaré) and played a Commander in the Sontaran Special Space Service named Stor in a 1978 episode of Doctor Who.

Elizabeth Spriggs Spriggs played the portrait of the fat lady guarding Gryffindor Tower in the Harry Potter films, and appeared in a 1987 episode of Doctor Who (where she is strangled by a cleaning device controlled by aliens.)

Jeff Rawle Rawle plays Amos Diggory in Harry Potter and the Goblet of Fire (father to Cedric Diggory), and played Plantagenet in a 1984 episode of Doctor Who titled Frontios.

Jessica Hynes Hynes provided the voice for Mafilda Hopkirk in the Harry Potter films before the character appeared in Harry Potter and the Order of the Phoenix, where she was played by Sophie Thompson. In Doctor Who, she played three separate related characters who The Doctor meets at various points in time.

Jimmy Gardner Gardner played the driver of the Knight Bus in Harry Potter and the Prisoner of Azkaban and played an alien named Idmon in a 1978 episode of Doctor Who.

John Atterbury Atterbury played the portraits of Phineas Nigellus Black in the Harry Potter films, and did some costume acting for several episodes of Doctor Who in the 1960s.

Julian Glover Glover provided the voice for Aragog in the Harry Potter films, and played both Richard the Lionheart in a 1965 episode of Doctor Who as well as an alien named Scaroth in a 1979 episode.

Terence Bayler Bayler plays the ghost and portrait of the Bloody Baron in the Harry Potter films, and appears as an alien named Yendom in an episode of Doctor Who from 1966.

Other Connections:

The actor that plays Dudley in the Harry Potter films, Harry Melling, is actually the grandson of Patrick Troughton, who played the second doctor.