Time travel

Time travel is, as the name suggests, the process of travelling through time.

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 Assasin). The Imperial Daleks under Davros, who already had limited time corridor 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.
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 conjuction 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, indiciating, perhaps, a similar function.

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. (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)

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)

By unexplained methods

Edward Waterfield and Theodore Maxtible attracted the attention of the Daleks while experimenting with static electricty and mirrors.
The mysterious Gateway situated in the void between N-Space and E-Space provided access different times and realities, sometimes through mirrors. Again, 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.