ColourBrewer2.0: Tool to help select appropriate colour palettes based on your project.
Charts, Graphs (& simple maps)
Datawrapper: Tool with an open tier for creating charts, maps and tables (I love the annotation functionality on this tool!).
Excel: Familiar tool, good for creating charts, graphs and dashboards. Try using Sparklines to get an overview of your data. Active community forum for support and instructional videos. Integrates with Office365.
Gephi: Open-source tool for network and system analysis to find new patterns. Steep learning curve but high quality results.
Google Sheets: Online app, part of Google Drive. Good for sharing data and creating charts and graphs.
PowerBI: Create sharable charts, graphs and dashboards. Free service, active community forum for support and instructional videos. Integrates with Office365.
RAWGraphs - Markets itself as the link between spreadsheets and visualization.
Tableau Public: Create sharable charts, graphs and dashboards. Free service, active community forum for support and instructional videos.
Piktochart: Create sharable infographics, free service, but sign-up required. Lots of free tutorials.
Visme: Create sharable infographics and charts. Free tier with limited storage and projects, has ability to export as .jpg.
ESRI Software (ArcGIS Pro, ArcGIS Online, StoryMaps etc). Proprietary suite of tools for doing GIS analysis and geospatial data visualization. There is a large forum of people online who post their questions and solutions.
QGIS: Open-source program for GIS analysis. Good entry point into geospatial analysis for those who do not need the in-depth tools and services provided by ArcGIS. Active community of support and learning materials.
Graphic Design (Open tools)
Inkscape: Open-source vector graphics editing software (equivalent to Adobe Illustrator).
GIMP: Open-source raster graphics editing software (equivalent to Adobe Photoshop).
Scribus: Open-source desktop publishing software (equivalent to Adobe InDesign).