Smilla Enlarger, which uses a special algorithm invented by the developer to re-size or enlarge an image (especially magnify bitmaps in high quality), is a portable application for Windows and Mac OSX.
Using this application is simple, select the file; settings and zoom region; adjust the parameters like De-noise, dithering etc, click preview and save your image.
While testing the software here, it gives a good result. You only require a few minutes to complete the task.
You can download Image Enlarger (Mac/ Windows, 8Mb Zip) from Sourceforge.net.