They are very versatile... I use them for the head on caddis pupa, hackle for midge dries, and for the top layer on streamers.
They can be pretty delicate, though, so wrap it around the thread before tying on...
Oh all kinds of stuff. Over wings on streamers, bodies on wooly buggers, gills on nymphs, tails on nymphs. Wrap for heads on caddis larva and pupa. Salt water flys. Look great on squid patterns.
The Catskill Coiler (AKA Curler) stonefly is my favorite use of ostrich herl in a pattern. Two part video:
http://www.youtube.com/watch?v=vS5OMH8np_0
http://www.youtube.com/watch?v=WyG2cY2QPo0