The following categories exist on the wiki, and may or may not be unused. Also see wanted categories.
- Waste (53 members)
- Waste heat recovery (1 member)
- Waste management (127 members)
- Waste to energy (7 members)
- Wastewater (48 members)
- Wastewater treatment (20 members)
- Water (397 members)
- Water Filter (1 member)
- Water Wheel (1 member)
- Water and sanitation for developing countries (37 members)
- Water conservation (47 members)
- Water disinfection (5 members)
- Water distribution (13 members)
- Water filters (35 members)
- Water filtration (8 members)
- Water heating (15 members)
- Water pollution (10 members)
- Water purification (99 members)
- Water quality (54 members)
- Water recycling (5 members)
- Water supply (45 members)
- Water treatment (41 members)
- Water use (1 member)
- Water videos (53 members)
- Waterborne diseases (3 members)
- Watering (1 member)
- Waterpod (19 members)
- Watershed Water Quality Monitoring Project (8 members)
- Wattle and daub (5 members)
- Weather (3 members)
- Web platforms (2 members)
- Webcams (1 member)
- Websites (4 members)
- Wellness (1 member)
- West Midlands (5 members)
- West Midlands files (4 members)
- West Midlands images (0 members)
- Western (0 members)
- Western Asia (8 members)
- Western University (10 members)
- WetLand (10 members)
- Wetland (3 members)
- Wetlands (3 members)
- Wheelchairs (4 members)
- Widget templates (8 members)
- Widgets (0 members)
- Wika (1 member)
- Wiki Green (4 members)
- Wikis (37 members)
- Wikis for human development (6 members)
- Wikis for international development (29 members)
- Wikis for social change (8 members)
- Wikis for sustainability (50 members)
- Wiltshire (2 members)
- Wind power (54 members)
- Windbelt (2 members)
- Windbelts (4 members)
- Winter (1 member)
- Wireless Communication (1 member)
- Women (9 members)
- Women's health (3 members)
- Wood (144 members)
- Wood working (6 members)
- Woodland and forest (1 member)
- Work exchange (3 members)
- Working animals (5 members)
- Workshop (15 members)
- World Shelters (10 members)
- Yorkshire and the Humber (7 members)
- Yorkshire and the Humber files (4 members)
- Yorkshire and the Humber images (0 members)
- Young and beginning farmers (2 members)
- Z-Plasty (25 members)
- Z-plasty (7 members)
- Zambia (6 members)
- Zane Middle School (57 members)
- Zef upcycling (6 members)
- Zero-point energy (4 members)
- Zero Hunger (1 member)
- Zero waste (4 members)
- 한국어 (2 members)