A short list of operational servers

UAF (Unified Access Framework - A GEO-IDE project)
NVODS (National Virtual Oceanographic Data System)
OSMC (NOAA's ocean Observing System Monitoring Center, with custom user interface )
PFEL (Pacific Fisheries Environmental Laboratory)
My Nasa Data (NASA data for educators)
AVISO (Archiving, Validation and Interpretation of Satellite Oceanographic data)

Here's a more extensive list