A curated collection of GIS datasets, maps, apps, and data layers. Living Atlas is a great first place to look for data because you can be sure the datasets are of high quality. However, the collection is relatively small (less than 20,000 datasets), so you may need to try other collections, too.
A Non-Curated Repository
Much bigger, but quality can vary so choose carefully
An evolution of the Geospatial One-Stop E-Gov initiative, this web site allows one to search or browse data categories of geospatial information. The GeoData Map Viewer allows you to view multiple Internet map services (like the National Map) using just your browser. Map servers viewable include the USGS, FEMA Hazard Maps, NASA Globe program, NOAA, and the BLM. State and regional servers are being added, including the Minnesota Metro GIS Data Finder.