hqx
A family of pixel-art scaling filters that compare each pixel with its eight neighbours and blend in smooth edges.
Maxim Stepin released hq2x, hq3x and hq4x in 2003; the number is the scale factor. Every source pixel is checked against the eight around it, and a neighbour counts as different if the gap in YUV space crosses a threshold. That yields 256 possible patterns, and for each one the filter looks up how the output pixels should be interpolated.
Unlike Scale2x, hqx invents new colours, which buys rounded outlines and antialiased diagonals. It became the default filter for a generation of emulators, and a reliable argument starter among purists who think pixel art should look like pixel art. Its successor xBR handles curves better.