YTimer2 is a flexible reminder program which can manage multiple timers, alerting you when they expire.
The program is very easy to use. If you just want a reminder to occur in 5 minutes, for instance, all it takes is a click on the "5m" button. An "Instant Timer" is added to the list, it starts counting down immediately, and once the five minutes are up then an audio alert will let you know.
If "5 minutes" isn't convenient then you can enter however many seconds or minutes you'd like to wait, or enter a particular time of day when the reminder should pop up. Give the timer a name and again it'll start counting down.
By default yTimer2 will just make a noise when a timer expires. This is effective enough - it sounds like a fire alarm, so is a great way to grab your attention - but if that's not appropriate then you can alternatively run the program of your choice. And optionally, you might also choose to repeat the timer, so for instance you could have it pop up an alert every hour.