The Sequencer alternates between each of four sides in sequence at a user-specified speed, sending out a Redstone signal as it passes each side. Since it is synchronized to world time, it also makes a handy clock - simply set it to 300 seconds. It can constantly switch on and off machines, pistons, etc.
Sequencers are synchronized to the world time. Consequently, any action which causes a change in the world time (like sleeping in a bed, or using the /time command) will cause the sequencer to change state. In some designs, this will have little-to-no effect, but if a sequencer is used to manage time-sensitive circuits, it may cause bugs in the circuit design.
Putting a sequencer next to a Lamp and connecting two of the facing outputs together will result in two cables where one will switch on and the other is then always off. This function may prove rather useful to power nuclear reactors and other dangerous machines that need to be switched off on a regular basis.