Adobe Lightroom 3 - this allows editing similar to what is possible in a darkroom. It is relatively easy to use. However, its strength is in its organizational abilities in addition to strong imaging editing (as opposed to image manipulation). I highly recommend and use this program.
Adobe Photoshop - this is an expensive and cumbersome program used by professional photographers and graphic designers. Just about any type of manipulation is possible using this program. Furthermore, there are hundreds of "plug-ins" that allow additional features. I do not use it because it is too cumbersome, complicated and expensive.