A dictionary of the English language, published by Houghton Mifflin Company.
A nice feature of the dictionary is the usage notes. Instead of succumbing entirely to prescriptivism, the dictionary has appointed a Usage Panel made up of writers and educators, who often vote on whether controversial grammatical constructions are okay or not, and in what circumstances. There are other interesting comments along these lines:
To emphasize that the probability of acquiring or spreading a sexually transmitted disease is merely reduced and not eliminated when following recommended safeguards, some advocate the use of the term safer sex instead of safe sex.
For those who enjoy etymologies, the AHD has appendices of Proto-Indo-European and Proto-Semitic roots that survive into the English language.
The fourth edition of the American Heritage Dictionary is on-line and searchable at http://www.bartleby.com/61/.