This is most clearly seen in Japanese Anime such as Akira or Arcadia of my Youth. Simply put the knowledge that people have, and the things they experience are genetically transmitted to their offspring. These memories are almost always dormant, but can be activated and relived. This theory has many applications, such as in dealing with cloning, and with the Knowledge is Power theory.
This theory assumes that, as it is called, Knowledge is Power. Literally. Either physical strength, or increased mental abilities. Combined with genetic memory, can result in unlimited power (does this explain Akira?)
[This is also used in martial arts films, Akira.]
Time is fixed, there is only one reality. The past cannot be changed, often attempts to change the past unintentionally create the events that the time travelers hoped to prevent in the first place.
Time is precieved as a single line.
Time is not constant, but is a series of overlaping "frames," which would allow time to be stopped.
Exception: History can be changed, but the future that the time traveler left no longer exsists... Only one future exsists at a time.
Exception: History can be changed, but it creates a temporary alternate universe that feeds off of the overall energy of the real universe, shortening its lifespan.
[Used in Doctor Who, Back to the Future, Babylon 5and others.]
There are an infinate number of realities/timelines. Every decision, choice, or chance possible exsists in one universe or another, so that all things are possible and that all things exsist. With that being the case it is therefore possible to go back in time to create change events for a different future... both futures still exsist, but to the time traveler he wiped out the first in favor of the second. Alternatively it is possible to move from one universe to another, visiting infinate variations of the same places.
[Used in Star Trek and Sliders... Maybe Quantum Leap aswell.]
Time is not fixed. Or Time is less fixed in certain areas than in others. This phenomina is hard to describe... Best phrased as time not working right. In some places time passes slower/faster than it should, or it may shift between too fast and too slow at random.
In other cases places exsist in all times, so that people from different eras can meet and have a drink together. Although in these places it is for some reason possible to visit it more than once, but never possible to meet yourself.
[Used in Doctor Who, Babylon 5, and Star Trek Books, and in the Hitchhikers series.]
This comes in two forms. The simple one is that you simply cannot exsist in the same time and place more than once, so if you meet yourself you could destroy all of creation. [Doctor Who]
A more complex form is that you can live time in any order, but you cannt relive the same part of time more than once. [Sticky Fingers of Time]
This is not so much as an actual theory as it is a bad plot device. This type of story centers on a character who is either from the past, or is so eccentric that they live in the past (drive an old car from the seventies instead of a modern electric car). The story shows how nice and wonderful the future is, and something happens that no one of that time can deal with... a crime that defies the central computer or some other such thing. The hero from the past uses some long forgotten skill (almost always taken for granted in the present) from the past to save the day. Buck Rogers saving the day through his advanced knowledge of Disco comes to mind as one such plot, although there are several others that were more watchable.
A character who simply likes or studies the past and implements some small modification (like compartmentalizing the Voyager based on the system used on Titanic) based on some of his research does not come under this theory, but something that is pivotal to the overall story (like killing a Zarg with a makeshift gun using bullets the character just happened to have left over from cleaning his great grandfather's antique pistol) would.