rolling thunder is done with a thick bar...and it sort of rolls when you grip it so it tests you.
http://www.juliehavelka.com/jpgs/sm_...2002_todRT.jpg
block pinching is just pinching two slabs together..you can vary number of fingers used. thors hammer is not really grip but more forearm
http://www.abcbodybuilding.com/exerc...horshammer.htm
edit: oops forgot topic....plate curls result in stronger wrist strength directly. the others are a less direct benefit to the wrist.