Observational:

• Cosmic Microwave Background:
• Richards Group (with MAXIMA links)
• Holzapfel Experimental Cosmology Lab (with ACBAR links)
• Smoot Astrophysics group
• BOOMERanG
• COMBAT: CMB Analysis Tools
• WOMBAT: Wavelength-Oriented Microwave Background Analysis Team
• Dark Matter:
• CDMS
• MACHOs
• DEEP Spectroscopic survey
• Pulsar, Gravitational Wave Background Group
• Supernova cosmology:
• Filippenko Group
• Katzman Auto-Imaging Telescope
• SNAP
• Supernova Cosmology Project

Theoretical:

• Theoretical Cosmology Group(includes other related theory links, some are below)
• Galaxy Clusters
• Large Scale structure, e.g. Galaxy Surveys
• Cosmic Microwave Background, primary anisotropies and secondaries such as the SZ effect
• Lyman-alpha Systems
• Gravitational Lensing (weak and strong)
• Dark matter dynamics
• reionization and 21cm radiation
• Numerical simulations

Other (reading groups etc)

• Institute for Nuclear and Particle Astrophysics and Cosmology
  
• The Berkeley Gravitational Lensing Group