Minecraft wiki entry for Clock, associated text: Hopper-Loop Clock
N-Hopper-Loop Clock Shown: 4-Hopper-Loop Clock.
Ethonian Hopper Clock (EHC) – Both pistons are sticky.
1-Wide Compact EHC
1-Wide Tileable EHC
1-Wide Upside-Down EHC
Non-stick EHC – doesn't require sticky pistons
Hopper Timer (self-locking EHC with trigger input)
RS NOR Latch Hopper Clock
1-Wide RS NOR Latch Hopper Clock
Hopper-Latch Hopper Clock
Sethbling's Hopper Clock
SethBling's Hopper Clock (Simplified)
SethBling's Hopper Clock (Amputated)
Multiplicative Hopper-Dropper Clock
3-Stage Vertical MHDC — 72 block volume, clock period up to 10.7 years