Trolling the internet leads you to some interesting websites. Here is one from a blog that features pictures of ten of the rarest cloud formations.
These are some of the most interesting pictures.
Mammatus Clouds: These clouds are formed by clouds sinking in air. They are most commonly seen after large thunderstorms.
Lenticular Clouds: These clouds are stationary lens-shaped clouds. They form at high altitudes and are normally aligned at right angles to the wind direction. They can sometimes be mistaken for UFOs.
Roll Clouds: These clouds occur at a low altitude and are usually associated with a clod front or the front of a thunderstorm.
Stratocumulus Clouds: These clouds occur at low altitudes and form long ribbon-like structures due to being trapped in air currents. While these cloud formations are not particularly uncommon, clouds formed into such distinctive ribbons, as in this picture, are very rare.
For more cool cloud pictures, check out the blog here.