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The Datamath Calculator Museum: Maintained by German engineer Joerg Woerner, this is one of the best sites on the 'Net for the history of Texas Instruments calculators.
Hewlett-Packard: Nonetheless a fine manufacturer.
        The HP Museum: Home of an outstanding display on the history of the HP product line, especially the old LED models.
Sharp and Casio: Maybe a cut below the big two in terms of market share, but they're still putting out some nice calculators.
Calculated Industries: A company that produces calculators for specialized applications--and will build you a custom calculator if you pay them enough!
Smithsonian Institution Chip Collection: Some images of rare and unreleased (and released and not-so-rare) TI products housed at The Institution.