The Communications Workers of America have a campaign that has been slowly winning some unionization efforts:
Overall union efforts:
Alphabet (Alphabet Workers Union)
Mapbox (Mapbox Workers Union)
Medium (Medium Workers Union)
Activision Blizzard (ABK Workers Alliance)
NPM (failed effort)
Kickstarter (Kickstarter United)
New York Times Tech (New York Times Tech Guild)
NPR Digital Media Division (Digital Media United)
Vodeo Games (Vodeo Workers United)
Raven Software (Game Workers Alliance)
Glitch
Voltage Entertainment
Blue Stage Digital (Blue State Union)
Mobilize (Mobilize Union)
Catalist (Catalist Union)
Change.org (Solidarity @ Change)
EveryAction (EveryAction Workers Union)
Top comments (1)
These folks are also helping with organizing: techworkerscoalition.org/
I found this organization as well, but it doesn't have any recent public activity: iww - iu 570.