HEASARC - NASA's High Energy Astrophysics Science Archive Research Center
Infrared Science Archive - NASA's science and data center for infrared astronomy
Extragalactic Database - NASA's archive of data for over 3 million extragalactic objects
Virtual Astronomical Observatory - Astronomical data from ground and space-based telescopes. Includes data analysis tools
National Space Science Data Center - Archive for NASA space mission data
Sloan Digital Sky Survey - Download optical images of the sky. See also, SkyServer for educational portal to the data.
Clean Energy Project Database "The Clean Energy Project Database (CEPDB) is a massive reference database for organic semiconductors with a particular emphasis on photovoltaic applications." (taken from databib.org)
The Cooperative Association for Internet Data Analysis "CAIDA" curates datasets resulting from both active and passive measurement of the Internet." (From their website)
DOE datacenters OSTI is the DOE official data-center for storing DOE funded research.
MatDB - A European Union based materials database.