Time travel

Time travel is, as the name suggests, the (usually controlled) process of travelling through time, even in a non-linear direction.

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The Doctor time travels. (DW: Day of the Daleks)

Methods

Technological or Biotechnological Methods

By space-time vessel

Time travel of this kind commonly requires a great deal of power. The Time Lords converted a star into a black hole in order to achieve time travel (DW: The Three Doctors) and used the Eye of Harmony thereafter (DW: The Deadly Assassin).

The Imperial Daleks under Davros, who already had limited time travel technology, expected to do the same. (DW: Remembrance of the Daleks).

Following the (presumed) loss of the Eye of Harmony, the Doctor used the Cardiff rift to "re-charge" his TARDIS. (DW: Boom Town)

Travel using time vessel commonly involves the Time Vortex.

Other Technological Means

Osmic projection, a means used by the Sontarans. The Doctor described this method as rather limited in its temporal range, when used by the Sontaran scout. (DW: The Time Warrior)
Time Scoop technology enabled the user to transport other people and objects remotely. But without any ally on the other end to return one, it may leave the user stranded in another time period with no way to return. (DW: Carnival of Monsters)
Time corridor technology used by both the Daleks (DW: Resurrection of the Daleks, Remembrance of the Daleks), Osirians (DW: Pyramids of Mars) and the Borad. (DW: Timelash)
A kind of time viewer called a quantum imager, enabled one to observe and (to a limited degree) communicate with people in the past. (BFBS: The Least Important Man)
Pushing energy into warp drive to create a time window. (DW: The Girl in the Fireplace)

A warp drive accident fractured one unfortunate individual, Scaroth into "fragments" scattered through various eras of time and linked by telepathy (DW: City of Death). Another warp drive accident had the effect of propelling the vehicle in question roughly 65 million years back in time. (DW: Earthshock)

The use of a rift manipulator in conjunction with the temporal rift itself. (TW: Captain Jack Harkness, End of Days)

The column in the center of a Human-built rift manipulator bore a resemblance of the central column a TARDIS, indicating, perhaps, a similar function.

A Time Agent Vortex Manipulator could be used as a crude time travel device, although it was known to misfire by hundreds of years and break down easily. (DW: Utopia, The Sound of Drums)

A time machine was created by UNIT with the help of Rose Tyler and the unusable TARDIS to send Donna Noble back to set history back on its true course. It was only ever used one time and is presumed erased from history along with the alternate world. (DW: Turn Left)

By Psychic Power or Other Natural Ability

The Eight Legs could teleport as easily through time as through space. K'anpo Rimpoche, a highly advanced Time Lord with great mental discipline, could do the same. (DW: Planet of the Spiders)
The enigmatic Bilis Manger could also also teleport at will through both time and space. This ability is only seen in the vacinity of the Cardiff Rift however, and so may be linked to it in some way.(TW: Captain Jack Harkness, End of Days)
Transcendental Beings have free movement through space and time.
Fenric could transport other living beings via time storms. The people displaced by the time storm often believed they had caused the time storm themselves. (DW: Dragonfire, Silver Nemesis, The Curse of Fenric)
The Fairies could travel through time and space naturally. (TW: Small Worlds)
The Weeping Angels had the ability to send others back in time with a touch. (DW: Blink)
The Trickster, prior to manifesting as a corporeal being, could traverse time and space with ease, though not at any given moment. (SJA: Whatever Happened to Sarah Jane?, The Temptation of Sarah Jane Smith)

By Space-Time Anomaly

The Tharils "rode the time winds". (DW: Warriors' Gate)
Possibly a warp ellipse can make time travel possible. (DW: Mawdryn Undead)
Travel via time rift is possible. The Weevils arrived on Earth via this method (TW: Everything Changes), as did a Human aircraft, the Sky Gypsy. (TW: Out of Time). During rare "negative spikes", the Rift also occasionally abducts random people in time and space from Earth, and diastrously attempts to correct itself. (TW: Adrift).
Sarah Jane Smith acquired an alien/futuristic Time Converter which allowed her to open and close time fissures, but these usually only link two particular places in time. (SJA: The Temptation of Sarah Jane Smith)

Mirrors

Edward Waterfield and Theodore Maxtible attracted the attention of the Daleks while experimenting with static electricity and mirrors. (DW: The Evil of the Daleks)
The mysterious Gateway situated in the void between N-Space and E-Space provided access different times and realities, sometimes through mirrors. The exact process remains unknown. (DW: Warriors' Gate)

Taking "The Long Path"

Some individuals, by necessity, may seek to take what the Doctor once described as "the long path" (DW: The Girl in the Fireplace) spending hundreds of thousands of years patiently waiting to return to a desired time. (NA: Birthright) A less arduous method would involve cryogenics or some other form of life suspension.