Academic Journals  

• Classical and Quantum Gravity
• Communications in Mathematical Physics
• Foundations of Physics
• General Relativity and Gravitation
• International Journal of Modern Physics D
• Journal of Mathematical Physics
• Living Reviews in Relativity
• Physical Review D
• Physical Review Letters 
• Physical Review Online Archive (1893-2001)

Other Resources 

• Hyperspace (International Society on General Relativity and Gravitation)
• General Relativity Group at Shanghai Normal University, Shanghai, China
• Albert Einstein Institute, Max-Planck Institute for Graviational Physics
• Perimeter Institute for Theoretical Physics
• Stephen Hawking Online
• Twistor Web
• Center for Gravitational Physics and Geometry, Penn State University
• Formal Geometry and Mathematical Physics, Utah State University
• Gravitation and Relativity Group, University of Cologne
• Matters of Gravity (Gravitational Physics Research Newsletter)
• e-print archive (Taiwan)
• e-print archive (arXiv.org)
• National Center for High-Performance Computing
• Geometric Algebra Research, Cambridge UK