Yeah...I'll disagree that EVH "invented" the hammer-on. Its been around a while- I've encountered some classical guitar exercises and songs that use it, and Jimi Hendrix and Roger Fripp definitely used it in rock before EVH did.
However, EVH definitely lifted the technique into prominence- before him, hammer-ons were primarily used to accent a short passage or to do something visually showy. EVH, OTOH, would play long, virtuosic solos and passages entirely with hammer-ons.
Some other guitarists to check out for expansive use of this technique would be jazz great Stanley Jordan and metal shredder Michael Angelo Battio- both use somewhat personalized versions of the technique with great flair.