Timelapse Toolkit is very helpful if you shoot timelapse with DSLR cameras (Watch the video at timelapsetoolkit.com).
 It offers tools like the timelapse calculator, which unlike others, is able to calculate different values. It can be used to calculate the total length of the clip, as well as the duration of the event, and of course the interval between the shots.
It comes with other features such as a torch, very handy when shooting at night, a Sun and Moon tracker, to know the time of sunset, sunrise, moonset and moonrise. Also includes a page with tips on how to get the best results in your timelapses.