I frequently recommend my colleague Ben Nortier's demonstration application Dayfindr. This is a simple calendar co-ordinator, allowing an arbitrarily large group of people to find a date or dates when everyone will be available to undertake some kind of joint activity, such as a meeting, theatre visit etc. Although it doesn't deal with subtleties like specific times of day, the comment facility is there to let you fine-tune your entries.
Briefly, Ben put this together as an exercise in Erlang / OTP programming and it's getting an ever-increasing amount of use simply through word-of-mouth recommendation. So far it has easily coped with everything that's been thrown at it. Every time I show it to someone, it knocks them sideways with its straightforward simplicity, robustness and speed, even though it is hosted on a virtual server with very frugal memory and processor allocations. Give it a try and see what you think!