The following are some sites we like to refer to. 

Ariel Investments John W. Rogers, Jr.
Baron Funds Ron Baron
BeEarly.com Interesting graphs and commentaries
Berkshire Hathaway self explanatory
Bogle Financial Markets Research Center Vanguard's Founder - John Bogle
BrandyWine Funds Foster Friess
Clipper Fund Value oriented, with long term investment horizon
Davis Funds Long term investors
Dreman Value Management David Dreman
Fairholme Funds Bruce R. Berkowitz
Georgia Tech - College of Management Interesting working papers
GMO Jeremy Grantham
J. V. Bruni and Company Good resource with book reviews
Legg Mason Value Trust Interesting to see what Bill Miller is up to.
Longleaf Partners Mason Hawkins, CFA
Martin Capital Management Interesting letters - very in-depth
Muhlenkamp & Company Ron Muhlenkamp
Northern Trust Nice commentary section
Oakmark Funds Bill Nygren
Oak Value Fund  
The Olstein Funds Robert A. Olstein
PIMCO I like to read Bill Gross
Sarbit Asset Management Larry Sarbit - interesting Buffett stuff
Sequoia Fund hall of fame fund for sure
Third Avenue Value Funds Marty Whitman
Tweedy, Browne Value Investors
West Coast Asset Management Founder of Kinko's
Weitz Funds Managed by Wally Weitz
Werlinich Asset Management I came across this, have no idea of performance, but enjoy his letter.
Zweig Funds Marty Zweig