Immanuel Velikovsky has an entry in Wikipedia. It starts out like this:
"Immanuel Velikovsky (June 10, 1895 (NS) – November 17, 1979) is best known as the author of a number of controversial books on prehistory, in particular, the US bestseller Worlds in Collision, published in 1950. Earlier, he played a role in the founding of the Hebrew University of Jerusalem, and was a respected psychiatrist and psychoanalyst.
"His books use comparative mythology and ancient literary sources (including the Bible) to argue that Earth has suffered catastrophic close-contacts with other planets (principally Venus and Mars) in ancient times. Velikovsky argued that electromagnetic effects play an important role in celestial mechanics. He also proposed a revised chronology for ancient Egypt, Greece, Israel and other cultures of the ancient Near East. The revised chronology aimed at explaining the so-called dark age of the eastern Mediterranean (ca. 1100-750 BCE) and reconciling biblical history with mainstream archeology and orthodox interpretations of Egyptian chronology. Indeed, revising or correcting (in his view) the conventional chronology of Egypt was his principal concern."
Velikovsky's theories have generally been rejected or ignored by the academic community.
In summary, Velikovsky believed that Jupiter expelled a huge comet, which eventually became Venus. The expulsion of this comet created the Red Spot that we see on Jupiter to this day. This comet nearly collided with Earth after it was expelled from Jupiter in about 1500 BCE, causing the plagues of Egypt, and the parting of the Red Sea, and nourished the Children of Israel with manna (which he says is made of hydrocarbons from the comet). That same comet then made the earth stop spinning when Joshua commanded the sun to stand still. The earth started spinning again. Then the comet zoomed to Mars, throwing Mars out of its orbit. Mars then nearly collided with Earth, destroying the army of Sennacherib. Mars settled into its present orbit, and the comet became Venus, settling into its own orbit.That’s quite a busy comet.
Velikovsky was Jewish and attempted to explain many of the events in the Old Testament. Velikovsky was also a contemporary and life-long friend of Albert Einstein – a man who knew quite a bit about the movement of planetary bodies.
Are Velikovsky’s ideas plausible? Here’s a response from a Physics professor:
"Could Velikovsky's ideas be true? How can we test them? If there were some obvious physical mechanism for moving planets around and changing their rotations the way Velikovsky claims, we might have reason to believe him. It would take as much energy as the Sun emits in a year to expel Venus from Jupiter. The normal laws of planetary motion are known well enough for us to send spacecraft to Saturn and beyond, arriving only a few miles off target and a few seconds off schedule after a trip of a billion miles and years in duration, but the normal laws of planetary motion will not move planets the way Velikovsky says they moved. Velikovsky postulated electromagnetic forces, but there is no known way such forces could originate in the Solar system. There are many thousands of known double stars in orbit around one another, but we have never seen any undergo the sort of violent orbital changes Velikovsky claims took place in our solar System. The laws of physics offer little encouragement to Velikovsky."
Velikovsky’s friend Einstein was one of those who was critical of Velikovsky’s ideas. As a friend he didn’t wish to offer insults, but in a letter from Einstein to Velikovsky dated July 8, 1946, Einstein wrote these words:
"I have read the whole book about the planet Venus. There is much of interest in the book which proves that in fact catastrophes have taken place which must be attributed to extraterrestrial causes. However it is evident to every sensible physicist that these catastrophes can have nothing to do with the planet Venus and that also the direction of the inclination of the terrestrial axis towards the ecliptic could not have undergone a considerable change without the total destruction of the earth's entire crust. It were best in my opinion if you would in this way revise your books, which contain truly valuable material. If you cannot decide on this, then what is valuable in your deliberations will become ineffective, and it would be difficult finding a sensible publisher who would take the risk of such a heavy setback upon himself."
Clearly Velikovsky’s ideas can’t be considered to be serious science.
In my own experience, many creationists know something about Velikovsky and they know that his ideas may explain some of the events in the Bible. Few of them seem to look for details. I have often asked creationists who mention Velikovsky's name to decribe his claims in their own words. They are routinely unable to do so. Clearly this is just another example of creationists not really wanting to know any of the details because they secretly know how unlikely they really are to be valid.
 From the web page http://en.wikipedia.org/wiki/Immanuel_Velikovsky referenced on September 28, 2007.
 http://www.uwgb.edu/DutchS/pseudosc/vlkovsky.htm, referenced on June 4, 2008
http://www.varchive.org/cor/einstein/460708ev.htm, referenced on June 4, 2008