GIMP (Gnu Image Manipulation Program) plugin for high quality image enlargement using the methods EANBQH a.k.a. Sharp (Exact Area Natural BiQuadratic Histosplines) and IBFNBQH a.k.a. Smooth (Interpolatory Box Filtered Natural BiQuadratic Histosplines, basically a variant of Natural cubic spline interpolation).
Linux source (install with gimptool) and Windows/OSX binaries available at the GIMP plugin registry.
There is also a self-standing C filter version of EANBQH available at the GIMP plugin registry. This version only supports 8 and 16 bit binary PPM images.
Programmers: Nicolas Robidoux and Adam Turcotte. Windows and OSX binaries provided by Francois Collard, Sylvie Alexandre of http://www.aljacom.com and GIMP Plugin Registry user semperit.