Big Book of Dashboards - great primer on dashboard development; examples are in Tableau workbooks but the concepts are widely applicable
Flowing Data - cross cutting blog that hits on design, tools, development processes and more
New series from Ken Flerlage - SQL for Tableau - highlighting how data viz developers can benefit from learning some basic coding skills
On being a better consumer of information:
Visual Trumpery slides from Alberto Cairo's lecture tour
The Functional Art by Alberto Cairo
Blogs on dashboard design tricks (primarily Tableau-focused, but ideas can be more widely applied):
Ryan Sleeper
Viz Wiz (Andy Kriebel)
Data + Science
Chart selection heirachies, for ideas on what chart to use when:
Visual Vocabulary
Data Viz Catalogue
Data Viz Project
Blogs on data viz best practices:
Policy Viz
Storytelling with Data
Ann K Emery
Evergreen Data
Inspiration for what to do (and not to do) in data viz design:
WTF Visualizations
Junk Charts
Makeover Monday (#makeovermonday on Twitter)
SWD Challenge (#SWDchallenge on Twitter)
Energy in America (Tableau dashboard example Amanda called out)