Somehow I'd forgotten to mention that Mythmere Games has released a MS-Word document entitled Monster Compendium: 0e for use with Swords & Wizardry. Matt Finch has converted a vast array of monsters to S&W's format, including many creatures from 3e D&D, in addition to beasties of his own creation. Weighing in at 60 pages and over 30,000 words, it's an absolutely impressive piece of work and one that I'm sure I'll get much use out of.
At present, the Compendium is intended solely as a tool for referees looking to use some of their favorite monsters with Swords & Wizardry. One day, it might become a polished book available for purchase. In the meantime, if you have some original monsters you'd like to submit, pop on over to the S&W forums and make a post. One day they might see publication in a hardcopy edition of this great bit of work.