Machine learning

• David Heckerman, Microsoft Research. Author of a very good tutorial on Learning Bayesian Network
• Leo Breiman
• Ron Kohavi
  

Computational biology

• Uri Alon, Weizmann Institute of Science
  
• Russ B. Altman, Stanford University
• Joel Bader, John Hopkins University
• Naama Barkai, Weizmann Institute of Science
• Gary Benson, Mount Sinaï School of Medicine
• Bonnie Berger, MIT
  
• Alexander Bockmayr, LORIA
  
• David Botstein, Stanford University
• Vladimir Brusic, KRDL
  
• Doug Brutlag, Stanford University
  
• Martha Bulyk, Harvard University
• Harmen Bussemaker, Columbia University
  
• George M. Church, Harvard University
• Philippe Cluzel, University of Chicago
  
• Barak A. Cohen, University of Washington in St Louis
• Hidde de Jong, INRIA Grenoble
• Sandrine Dudoit, UC Berkeley
  
• Sean Eddy
• Nir Friedman, Hebrew University
  
• Terry Gaasterland, Rockfeller University
• Olivier Gascuel, LIRMM
  
• Mark Gerstein, Yale University
• David K. Gifford, MIT
• Walter Gilbert, Harvard University
  
• David Haussler, UCSD
• Susan Holmes, Stanford University
• Trey Ideker, Whitehead Institute, MIT
  
• Tommi Jaakkola, MIT
• Inge Jonassen
  
• Richard Karp, University of California at Berkeley
  
• Jens Lagergren, Stockholm Bioinformatics Center
• Hao Li, UCSF
  
• Jun Liu, Harvard University
  
• William Noble, University of Washington
• Lior Pachter, University of California at Berkeley
• Bernhard Palsson, UCSD
  
• Pavel Pezner
• Yitzhack Pilpel
• François Rechenman, INRIA
• Isidore Rigoutsos, IBM Research
  
• Edward M. Rubin, University of California at Berkeley
  
• Andrey Rzhetsky, Columbia University
  
• Marie-France Sagot, INRIA Grenoble
• Chris Sander, Memorial Sloan-Kettering Cancer Center
  
• Ron Shamir, Tel Aviv University
• Eric Siggia, Rockfeller University
• Mona Singh, Princeton University
  
• Donna Slonim
  
• Gary Stormo, University of Washington in St Louis 
• Saeed Tavazoie, Princeton University
  
• René Thomas, Université Libre de Bruxelles (works on mathematical modelling of gene regulatory networks)
  
• Martin Tompa, University of Washington
• Esko Ukkonen, University of Helsinki
  
• John Weinstein, NCI
• Wing H. Wong, Harvard University
  
• Michael Q. Zhang, CSHL
  

Biology / Medicine

• Patrick B. Brown, Stanford University
• Todd Golub, Whitehead Institute, MIT 
• Jules Hoffman, CNRS. Immune system in drosophila.
  
• Leroy Hood, Institute for Systems Biology. Read this interesting interview.
• Miroslav Radman, Inserm, studies DNA repair.
• Michael Snyder, Yale
  
• Rick Young, MIT
  

Psychology, Brain science, Scienctific reasoning

• Phil E. Agre, UCLA
  
• Kevin Dunbar
• Duncan Watts, Columbia University
  

Mathematics and Computer Science

• Christos Faloutsos,  DTW, data mining (has published an interesting paper at SIGMOD94: Fast Subsequence Matching in Time-Series Databases)
• Steven Lawrence Senior Researcher at Google
• Marvin Minsky, MIT
• Seymour Papert, MIT
  
• Ron Rivest, MIT
• Rob Tibshirani, Stanford University
  

Others

• Ray Kurtzweil
• Paul Krugman
  
• Edge.org
• Internet pionneers, ibiblio.org (Vinton Cerf, Marc Andreessen, Tim Berners-Lee, etc)
• Alan Turing
• Jon Udell (has some interesting pages on scientific collaboration tools)
  
• John Von Neuman
  
• Steven Wolfram, creator of Mathematica. See also http://mathworld.wolfram.com/, a very good site on mathematics at wolfram.com
• Scientists and thinkers, Time 100