There are fewer examples of dedicated upscaling software on the market than there used to be, with perhaps the need for such programs decreasing.
The most popular two cross-platform programs are Ben Vista’s PhotoZoom Pro (a reincarnation of the Dutch software S-Spline), and Genuine Fractals.
Genuine Fractals is not, strictly speaking, dedicated to upscaling, but was developed as a scaleless image file format. Nevertheless this is probably its most valuable use. What both have in common is that the algorithms they use for interpolation are much more complex than that for Bicubic—the Photoshop default for photographic images.