Gnome-schedule is a graphical front end for the 'crontab' and 'at' tools. See the homepage for more information.
Amazing project! I am using it for some basic tasks and I am very happy with it. Just one improvement suggestion, sometimes you need to organize the tasks in a different order in the list, so it would be great if the tasks could be dragged or reordered in the list. (Some programs have some arrows to move them up or down). Anyway, well done!!!
Very useful app, especially in case you find GUIs convenient!