National Phenology Network: Phenology refers to key seasonal changes in plants and animals from year to year—such as flowering, emergence of insects and migration of birds—especially their timing and relationship with weather and climate.
USDA VegScape: From the National Agricultural Statistics Service of the USDA, this tool allows you to view vegetation indices in a GIS format for years back to 2000. The GIS format allows you to zoom far into the county level, get statistics for the chosen area, and output into a pdf file.
NOAA STAR Global Vegetation Health Products: The Center for Satellite Applications and Research at NOAA has a tool with 17 vegetation health products, ranging from Vegetation Heath and Greenness to Fire Risk and Thermal Stress. Output is a large image in png format.