Spikes on LT1301 DC-DC converter
The "spikes" are the ringing in the inductor (self-resonance) associated with the switch-off transient of the regulator's switching transistor, coupled through the capacitance of the rectifier diode. There is actually very little energy associated with them, and they can be reduced to negligible levels by good circuit layout and some judicious bypassing of the output with a ceramic capacitor. This keeps the ringing from affecting other circuits via either conduction or radiation.

In extreme cases, a "snubber" (resistor and capacitor in series) connected between the switch node and ground is required to suppress the ringing. The component values required in the snubber depend on the specific characteristics of the inductor.