Doodle
Despite all the modern conveniences (and increasingly often because of them) of life and work there are many times when we have to waste time because of an inefficient system. One example is scheduling a meeting with multiple people. With two people it's pretty easy to figure out the intersect of schedules to see when both parties are free. With three people there may be a little back-and-forth, but it usually works out. Once you get to five people, it can be difficult or impossible. Doodle takes care of that. It's good.
Doodle is one of those sites that I kick myself for not having thought of. The back-end must be pretty straightforward. I can imagine re-creating the basic functionality in PHP or ColdFusion in an afternoon. But they thought of it and implemented it first. Good for them.