Timelapse Calculator was designed to help one decide the appropriate interval settings for timelapse applications. The unique thing about Timelapse Calculator compared to others you might find online is it's user specified dynamic interface.
This allows one to input the data that they already know, for example, the actual time you wish to allocate to shoot the timelapse sequence. Then the Timelapse Calculator does the rest of the thinking (or suggesting) for you. This allows for quick intuitive calculations so you can get the shots you want before the scenery changes and the moment passes.
iPhone Version (New)
Thanks Simon Loffer for his slick, and much more portable take on the Original Timelapse Calculator. Follow this link to download the iPhone app.