creating a gradient transparency effect

how would i be able to increase a layer's transparency as it goes from on end or the other, either being left to right or top to bottom

thanks

You can do this easily using a Layer Mask
First take the image you want the transparent effect on and put it on a layer other than the background.
Next, click the Add a mask icon on the layers palette. It's 2nd from the left and looks like a gray rectangle with a white circle within.
Next, use the gradient tool to make a Linear Gradient from black to white where you want the transparency effect. Where the gradient is black, your image will become transparent. As the gradient changes to white, the image will appear more and more opaque.
Still have questions? See my tutorial, Blend Images With A Layer Mask.