Ice

Ice has become more important recently in drinks. Different shapes and styles are de facto in the cutting edge mixologist's repertoire.

Styles:

Twice Frozen (or many times frozen) achieves a similar goal to heating before freezing. The air that is in solution with the water comes out of solution and allows the ice to freeze into a more dense, harder to melt block. This is useful for martinis and other drinks that are best served cold, but should not be watered down.

Mineral/Bottled Water can impart a different flavor in the water. This is usually used in citrus/acidic drinks where the melted water can mellow the drink out.

Shapes:

A big block of ice allows pieces to be chipped off in the appropriate shapes and sizes. Spears can be cut to fit into the shaker for stirring a martini, or small globes can be cut to fit into a tumbler.

Larger-than-usual cubes are useful to keep a rocks drink cold but not melt too much, this can help a scotch/rocks keep its flavor while imparting a minimal amount of water into the drink.

Source: wikibooks.org - the open-content textbooks collection