I was so impressed with Maxim Stepin's hq3x and related algorithms for scaling up sprite art that I ported his code to Linux and wrote a pnmhqxscale wrapper around it to use with the netpbm tools. The port is a hack, but it works OK. You can download it for yourself.
pngtopnm s.png | pnmhqxscale -3 | pnmtopng > 3x.png
  2003-11-01 18:33 Z