A sentence or phrase that, while technically correct, states the same thing twice and then equates them, thus conveying no meaning: "If you can't find what you're looking for, you're not looking in the right place." (Check for tautology if you find yourself tempted to reply "No $hit, Sherlock!")
Less severe: A sentence that conveys meaning but is unnecessarily redundant, as "I saw it with my own eyes".
Sometimes place names where the proper name is taken from a foreign language and the geographical feature part are in English are redundant. "Laguna Lake", "Glendale".
Essentially the opposite of an oxymoron like "Microsoft Works".
Hey, Tax-chick, check out #43 for some good tautologies.
The ones I could think of are: "Survival of the fittest" (because "fittest" is implicitly defined as "those who survive," so it essentially means, "Survival of those who survive.")
And the old stand-by, "A true Scot does not put sugar on his oatmeal," in all its variations. ("But Angus MacPherson of Aberdeen does!" -- "Ach, but then he's no true Scot!")