The list below is not in any way comprehensive or universal – I include only the tools I use and can vouch for their awesomeness.
- https://roll20.net/ – best virtual tabletop for online play
- https://shop.niktto.pl/ – Magical Shop generator with sane prices (some text in polish)
- https://www.dndbeyond.com/ – monster and spell listings with ability to create and save
- http://inkwellideas.com/ – producers of Hexographer, they offer a lot of tools for mapping (some free)
- https://kobold.club/ – easy to use encounter calculator
- http://www.dmsguild.com/ – a lot of really great adventure modules, homebrew classes, and resources.