Research Experiences for Undergraduates at the University of Bremen
arXiv (repository of electronic preprints)
XKCD (webcomic of romance, sarcasm, math, and language)
Numberphile (video podcast about mathematics and 'just' numbers in general)
OEIS (online encyclopedia of integer sequences)
3blue1brown (math YouTube channel)
PDE2PATH (Matlab package for continuation and bifurcation in systems of PDEs)
AUTO-07P (continuation and bifurcation in ODEs and much more)
DMV (german mathematical society)