KAlarm
KAlarm is a free and open source program for setting reminders, as well as for scheduling command launches and sending emails
Getting Things Gnome (GTG)
Getting Things Gnome (GTG) is a free and open source personal organizer/task manager inspired by the “Getting Things Done” (GTD) methodology
hmtimer
hmtimer (Hsiu-Ming’s Timer) – graphical computer shutdown timer and more
Go For It
Go For It – free and open source software for creating to-do lists with a built-in timer