PDE Simulator: https://visualpde.com/
Open problems in nonlocal PDEs: https://web.ma.utexas.edu/mediawiki/index.php/Open_problems
Seventy Five (Thousand) Unsolved Problems in Analysis and Partial Differential Equations (Integr. Equ. Oper. Theory 90, Article no. 25, 2018) by V. Maz'ya: https://doi.org/10.1007/s00020-018-2460-8
Mathematics Going Forward (Collected Mathematical Brushstrokes) https://link.springer.com/book/10.1007/978-3-031-12244-6