Minecraft wiki entry for Traps, associated text: Place lava in front of the dispenser to catch the target on fire.
You can adjust the repeater, which will change the speed. You can also completely change the clock if you don't have quartz.
You could use this to knock the target into a pit.
Attach this to a RS-Latch to keep it on until a button is pressed.
Use tipped arrows with negative effects to make the trap more dangerous.