Forest & Kim Starr
Lots of pictures of insects and plants, especially non-native species.
Gerald Carr's plant native plant pics
More pictures of native plants.
Hawaiʻi Statewide GIS Program
A very useful site with lots of free GIS data.
National Park Service, Inventory and Monitoring Program
USGS Geographic Names Information System (GNIS)
Useful for looking up names you don't recognize.