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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.  This link brings you to the sales page for recent HP models.
        The HP Museum: Home of an outstanding display on the history of the HP product line, especially the old LED models.
        HP-calculators.com: A new site with details on HP's 02002-02003 resurrection of its calculator business.  Still a bit embryonic, but a lot of collectors and aficionados are standing by.
Sharp and Casio: Maybe a cut below TI 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!  (See my collection of CI Calculators here.)
Kinpo Electronics: This calculator manufacturer produces calculators for Citizen and Ibico, and is currently making the newest Hewlett-Packard calculators (the HP-9S and HP-9G) as that company tries to reactivate its calculator division in some small way.

Smithsonian Institution Chip Collection: Some images of rare and unreleased (and released and not-so-rare) TI products housed at The Institution.