My favorite type o' clouds... at my favorite time o' day:
Mammatus clouds are fairly rare... they're also associated with severe weather. Both images were taken with my SLR with the 50mm lens mounted, rather than the grab-shot camera. The first pic was taken in full-auto mode, the second in shutter-priority mode at f1.4 and 1/320th of a second. The second shot is much closer to the true color of the sky.