Just about any photo editing software, including that which is provided with just about any digital camera, will allow you to resize a digital photo in this way. The methods by which this are performed in each program may be different, but usually there is a "resize" option. With most programs the resize option has a setting named something like "maintain aspect ratio" or something similar that instructs the program to keep the width to height ration constant while changing the dimensions of the image. Most also have a crop tool that allows you to cut away portions of the image that you don't want.

Most online photo storage sites also provide free online tools to do this too. This includes photobucket, smugmug, Costco.com, photoshop.com, and others.

You can also download free programs like Picasa or Gimp.