Simple. Easy to use. Graphic. Nice features for a service that offers event creation and coordination. WhichDateWorks provides all of that without the need for a sign-up process. Simply pick a date range, enter the email addresses of the desired attendees and wait. The recipients follow a link to the site and place their votes for the best dates for them.
I got the tip from Lifehacker, which also mentions another, similar service, When Is Good, with greater granularity as to time selection.