Minecraft wiki entry for Hopper_clocks, associated text: There are a number of useful outputs from this clock:
Clock: A regular on/off clock signal can be taken from one position of the block of redstone. The signal will last for half the clock period.
Cycle Off-Pulse: Either block faced by a comparator stays powered most of the time, but will turn off for 3.5 ticks every full cycle (but at half-cycle intervals from each other). The power level of the block may vary, so an output repeater may be needed to keep the power level constant.
Cycle Pulse: By placing a torch on one of the blocks powered by a comparator, the off-pulse is turned into a regular 3.5-tick on-pulse, once per cycle.
Half-Cycle Off-Pulse: By placing two redstone dust alongside or under the positions of the block of redstone, a 1.5-tick off-pulse is generated every half-cycle when the block of redstone moves.
Multi Clock: by attaching 4 interconnected hoppers next to the redstone block, every full cycle will run an item(s) through these hoppers once, before stopping by the redstone block. Attaching a comparator and then a repeater to the other end of these added hoppers makes a signal that is 1 tick on, N×2-1 tick off, where N is the amount of items in the Ethonian clock. The amount in the clock×2 equals the total clock ticks. The amount of items in the 4 hoppers determines how much of that time is powered.