It's functionally very close to https://colorbrewer2.org, which has been online for more than 15 years. I think it's not a coincidence, and some credits wouldn't have hurt anyone...
Also, I have personally been using iWantHue to generate large palettes for more than 10 years now, and I'm still very happy about it:
https://medialab.github.io/iwanthue/
I keep this in mind. We need to find an easy way to export examples formatted with the good template etc... before putting anything public, to ensure it's not too hard to update the production server when we release sigma.js.
Actually, as with d3, you can develop your own network visualization layouts.
The main difference is that sigma only deals with graphs drawing, so it deals without effort with drag and drop (mouse or touch), scaling the graph to the screen, smooth zooming (with mousewheel or pinch)...
Also, canvas is faster in Chrome. For instance, here is one benchmark about layered canvases (sigma.js actually layers different canvases): http://jsperf.com/layered-canvases/9
So, it is actually "best viewed in Google Chrome", despite it is indeed standards compliant.
I am formerly a big fan and user of Gephi, so I think graph drawing is "important enough" to deserve its own JavaScript library, which is why I made sigma.js. I know d3.js is already a great tool, but it looked hardcore for me to tweak it to draw graphs on a Canvas element (it is more easy to use SVG elements). And basically, using a Canvas makes it much more scalable.