Magic Find, or MF, is a very popular magical property found on many types of items. Magic Find has been confirmed to return in Diablo III, and it was seen on numerous items in the Blizzcon demo, as of November 2008.
Exactly how Magic Find will work in Diablo III isn't yet known, but it's assumed that it will work much as it did in Diablo II. In that game Magic Find increased the odds for found items (from monsters or objects) to be "higher quality." Higher quality items were more likely to be unique, set, rare, or magical, with different odds for each type of item depending on the level and type of monster dropping the item, the player's equipment, the level of the area the item was found in, and much more.
Magic Find was hugely popular in Diablo II, andgoes into exhaustive detail in discussing this complicated item property.