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Linotype's FontExplorer X (FEX) is a free font management tool, which includes an integrated font store. Users of FEX may purchase fonts via this store if they so choose (unlike traditional internet font shopping, fonts purchased via FEX will be directly installed in the font manager, for easier activation and deactivation).
As of this writing (April 11, 2006), FontExplorer X is currently available just for MacOS X 10.3.9 and higher. A PC version is in the works.