One day I wondered why the image processing people don't use a sinc() function to upsample images. The most common forms of upsampling in use are bilinear, and cubic. Both of these methods leave quite a bit to be desired, visually.
So, I wrote a program to use the sinc() function to upsample images. Here's the source code.
You should also take a look at my homepage as well as my Gimp & PhotoShop Image Enhancement Tricks Here are some test images: