A site I've been using a fair amount recently is called "When is Good?". This site is amazingly useful, and I figured I should spread the word about it.
Suppose you are trying to find a good meeting time for a reading group you are organizing, or that you are, perhaps, scheduling your dissertation defense. The "When is Good" site gives you a really easy way to collate the information you want about everybody's schedules. You enter some basic information about the event and the range of dates you are looking at, and it will give you a link to distribute to all of the potential participants. All they have to do is fill out their availability in that range. You get a separate page to look at that collects all the results together, and shows you which times work for everyone. It also tells you how many people have conflicts for any given time, and lets you exclude individual respondents to see what times work best for subsets of the groups.
It is free to use the site (you don't even need to register an account), though you can apparently join as a premium member and get some helpful add-ons (like giving people a way to indicate whether a time that they are available is ideal or not).