TNG
TOS's McCoy in Encounter at Farpoint.
TOS's Sarek in Sarek and Unification.
TOS's Spock in Unification.
TOS's Scotty in Relics.
DS9's Bashir in Birthright.
DS9's Quark in Firstborn.
(O'Brien reappeared in the series' finalé, All Good Things..., after joining DS9)
DS9
(TNG's O'Brien and Worf (from Season 4) are series regulars, while Keiko and Molly O'Brien recur semi-regularly).
TNG's Picard in Emissary.
TNG's Duras sisters in Past Prologue.
TNG's Q in Q-Less.
TNG's Lwaxana Troi in The Forsaken, Fascination and The Muse.
TNG's Admiral Nechayev in The Maquis Part II and The Search Part II (sort of).
TOS's Kor in Blood Oath, The Sword of Kahless and Once More Unto the Breach.
TOS's Kang in Blood Oath.
TOS's Koloth in Blood Oath.
TNG's Gowron in The House of Quark, The Way of the Warrior, Broken Link, Apocalypse Rising, By Inferno's Light, When It Rains... and Tacking Into the Wind.
TNG's Riker (sort of) in Defiant.
VOY's Tuvok (sort of) in Through the Looking Glass.
TNG's Kurn in Sons of Mogh.
TNG's Toral in The Sword of Kahless.
The TOS crew (archive footage, minus Sulu) in Trials and Tribble-ations.
TNG's Alexander Rozhenko in Sons and Daughters and You Are Cordially Invited (not the original actor).
VOY
DS9's Quark in Caretaker.
TNG's Q in Death Wish, The Q and the Grey and Q2.
TNG's Riker in Death Wish.
TOS's Sulu, Rand and Kang in Flashback.
TNG's La Forge in Timeless.
First Contact's Borg Queen in Dark Frontier, Unimatrix Zero and Endgame (although in the first two of these appearances she was not played by the original actress).
TNG's Reg Barclay in Projections (sort of), Life Line, Inside Man, Author, Author and Endgame.
TNG's Deanna Troi in Pathfinder, Life Line and Inside Man.
ENT
TNG's Riker and Deanna Troi in These Are the Voyages.
FILMS
TOS's Kirk, Scotty and Chekov in Generations
The Enterprise-E's EMH (VOY) and the USS Defiant in First Contact. Zefram Cochrane first appeared in TOS's Metamorphosis, played by another actor and with very different characterisation.
VOY's Janeway as an Admiral in Nemesis.
While not strictly direct character crossovers, Brent Spiner plays Arik Soong, an ancestor of Data's creator, Noonien Soong, in ENT's Borderland, Cold Station 12 and The Augments, while Michael Dorn plays an ancestor of Worf in The Undiscovered Country and Robert Picardo appears as Dr. Lewis Zimmerman, creator of VOY's EMH in DS9's Doctor Bashir, I Presume?.
