• Adobe color wheel - One can choose complementary colors with this tool.
• Refractive index info - One can find the refractive index of kinds of materials.
• Latex equation editor (online) - There is a detaited doc of Latex equation languages.
• WebPlotDigitizer - Extract data from images of plots, maps etc.

Interesting research outside my field

• Egg and math: introducing a universal formula for egg shape, by Valeriy G. Narushin, Michael N. Romanov, and Darren K. Griffin. pdf