I've used this a bunch! I found it invaluable to helping me think through the timing of FPGA components. The best part is you can embed it in your own web page: <a href="https://lkesteloot.github.io/alice/alice4/fpga-rasterizer.html" rel="nofollow">https://lkesteloot.github.io/alice/alice4/fpga-rasterizer.ht...</a>
plantuml does something similar using a syntax that you can actually write by hand:<p><a href="http://plantuml.com/timing-diagram" rel="nofollow">http://plantuml.com/timing-diagram</a>