Minecraft wiki entry for Quasi-connectivity, associated text: The following redstone components can activate mechanism components one block away, and update redstone components adjacent to the block they are attached to (including above and below) as well as redstone components adjacent to themselves:
Buttons (attaches in any direction)
Detector Rail (attaches only downward)
Lever (attaches in any direction)
Pressure Plates (attaches only downward)
Trapped Chest (doesn't actually attach, but updates as if attached to block beneath)
Tripwire Hook (attaches only sideways)
Weighted Pressure Plates (attaches only downward)