Joi Ito made a post a few days ago about writing style and named his favorite resource, The Chicago Manual of Style. He also lists some web resources, and remarks that "[p]oor writing style, like bad manners, makes someone appear less intelligent than they are."

I own a copy of Strunk and White (3^rd edition), and I am sure it is apparent that I seldom consult it. For usage, I adhere to Fowler's Modern English Usage, both for the substantive content and Fowler's overall crankiness. According to reader reviews at Amazon, R. W. Burchfield's edition has disappointed many who were accustomed to the Fowler treatment.

For a more recent tome on the topic of usage, the 2^nd edition of Hayakawa's Choose the Right Word comes highly recommended by several individuals.