SmillaEnlarger is an Open Source program that enables you produce quality enlargements of your digital photos. It enables you to convert a low resolution image into a high resolution copy while maintaining as much of the details as possible and minimizing image noise. You can also clip a portion of your photo and create a high quality crop with the same or higher resolution than the original.
SmillaEnlarger offers several adjustment options and an instant preview to inspect the results. A Help document comes included with the download, you`ll probably want to take a look at it before you get started.