Sometimes high altitude ice particles can catch the light long after the sun has set, as seen in the photograph above - these are known as noctilucent clouds. This phenomenon is most common near the poles in the summer, when the sun can only dip a short distance below the horizon while the upper atmosphere is still in daylight.
More common are iridescent clouds, which can occur during daylight. High altitude ice particles cause the light to be refracted, causing rainbow-coloured patches to appear if seen from the correct angle. The photograph below shows an example. Underneath are nacreous clouds, a similar phenomenon normally only seen near the poles. These ones were spotted from my sitting room window in winter.
Crepuscular rays occur when the sun is low on the horizon, shining up behind clouds as in the picture below.
Contrails are man-made clouds created by the exhaust of high-flying aeroplanes. They appear if the moisture levels are such that water vapour in the exhaust condenses to create a cloud. How long the contrail persists depends on the temperature and humidity of the air. In the picture below the contrail formed into clumps. Some plane spotters claim to have seen contrails like this being created by very fast and loud planes, and believe them to be evidence of secret aircraft projects. It has been suggested that a theoretical engine called a Pulse Detonation Engine might create such an effect. The one I photographed was caused by an ordinary airliner, however.
A couple of times I have seen unusual contrails like the one in the first photograph below. It shows an ordinary contrail almost aligned with a dark line. I'm not sure what caused this. One possibility is that the there was a thin layer of vapour below the plane and its contrail was casting a shadow. Another is that an earlier plane had passed by and it had caused existing moisture to condense - this is known as a distrail, a "dissipation trail". The second picture below shows a spectacular example of dissipation, where a passing plane appears to have caused a huge patch of cloud to collapse in on itself.
In thundery weather, sinking air can sometimes form into hemispheres underneath cumulus clouds, as seen below. These are known as mammatus, from the Latin word for mammary. They can look particularly striking when the sun is low in the sky, so that they are well illuminated.
One unusual cloud I failed to photograph was a green one caused by volcanic dust, originating in Alaska if I remember correctly. I couldn't go outside to take a picture as I was running a bath at the time. And naked.
Much more information on these cloud types is available online, at Wikipedia for example.