Design tools
Dextrous
Easy to you browser based tool for creating board game components. Desings exportable as PnP PDF-files as well as Tabletop Simulator and Screento.gg files.
Dextrous can be integrated with Google Sheets spreadsheets so that details like values and image references of the items (cards, tokens, etc) are maintained in spreadsheet and Dextrouds reads the values from there.
Both free and paid plans available. Free plan is limited to 2 projects (games), 10 item layouts and 100MB of storage. Paid plans starting from $48/year.
nanDECK
Free card deck creation tool for Windows. Relays hevily on card data written in spreadsheets.
Tabletop Creator (Pro)
An application for creating various board game elements suchs as cards, boards and tokens. Can use various spreadsheet formats as item datasource though does not require one.
Items can be exported as PnP PDF-files (with lots of different options in Pro) as well as Tabletop Simulator files that can also be used with screentop.gg. No direct integration to screentop.gg or Tabletop Simulator though and requires some work to import data to these environments.
Pro version available on Steam with a price tag of about 100€.
Digital prototyping
Tabletop Simulator
Also known as TTS. An application for playing (and creating) digital versions of board games. Almost an industry standard for digital playtesting.
Available from Steam with a price tag of about 20€.
https://www.tabletopsimulator.com
Screentop.gg
Web site for playing (and creating) digital versions of board games.
For creators free and paid plans available. Free plan is limited to creation of 3 games and 32MB storabe per game. Works somewhat decent also on mobile devices. Paid plans staring from $10/month.
Graphic tools
GIMP
Free alternative to Photoshop. Great tool for creating and editing images in a lot of different formats.
Inkscape
Free opensource tool for creating and editing vector graphics. Great for any SVG file creation for example icons.