[2014, JS] Works in browsers with WebGL enabled. Uses GLSL mainly for the path tracing, and has fairly realistic light transport (not currently spectral or polarized).
[2008, C++, Linux] Highlights common human-understandable patterns to eliminate candidates, using strategies up to swordfish/y-wing/coloring. Includes a command-line solver that outputs postscript or SVG step-by-step solution guides, and includes a GTK+/Cairo user interface for interactive solving.
Using edge-cases in HTML5 Canvas implementations, it's possible to create images that are different on most browsers without querying any browser information (via browser detection or reading Canvas pixels).
Not only does this allow creating unique images (try viewing the image to the left on other browsers), but it's possible to craft the ability to decide what to show for each browser.