use animation sheet like this

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and script is

//vars for the whole sheet

var colCount    : int =  4;

var rowCount    : int =  4;

//vars for animation

var rowNumber   : int =  0; //Zero Indexed

var colNumber   : int =  0; //Zero Indexed

var totalCells  : int =  4;

var fps  : int = 10;

var offset  : Vector2;  //Maybe this should be a private var

//Update

function Update () 

{

SetSpriteAnimation(colCount,rowCount,rowNumber,colNumber,totalCells,fps);  


}


function SetSpriteAnimation(colCount : int,rowCount : int,rowNumber : int,colNumber : int,totalCells : int,fps : int)
{

    // Calculate index
    var index : int = Time.time * fps;
    // Repeat when exhausting all cells
    index = index % totalCells;

    // Size of every cell
    var size = Vector2 (1.0 / colCount, 1.0 / rowCount);

    // split into horizontal and vertical index
    var uIndex = index % colCount;
    var vIndex = index / colCount;

    // build offset
    // v coordinate is the bottom of the image in opengl so we need to invert.
    offset = Vector2 ((uIndex+colNumber) * size.x, (1.0 - size.y) - (vIndex+rowNumber) * size.y);

    renderer.material.SetTextureOffset ("_MainTex", offset);
    renderer.material.SetTextureScale  ("_MainTex", size);
}

If you are using the same material multiple places, then going to a "texture atlas" solution will likely reduce draw calls and potentially improve performance. There are a number of different scripts around that implement this functionality both on UnityAnswers and in the Wiki. They animate the UV coordinates of the material. Here is one link:

http://wiki.unity3d.com/index.php?title=Animating_Tiledtexture-_Extended

Note third-party tools like EZGUI and probably NGUI automate the process of creating animated textures and building the atlas. That is, the frames exist as individual textures in the editor, but the tools build a single atlas/texture which is used at runtime.

To to be clear, in order to get a significant win out of this, you need to have multiple animations using the same animation sheet (and therefore they both would have exactly the same material).