Thats right, list parts you think have made a big dent on the history of trials, with reasons why. Try keep it fairly revolutionary, not "Tensile - Like Middleburns but cheaper"
Koxx Levelboss - Not only did this bring around the UCI craze, when it came out people were all "woah, this stupid long bike with no seat!" Bearing in mind it was only 1065 too. I remember people saying unless you were good, the Koxx would hold you back because of the extra length. However, soon everyone had one or copied the geometry.
Magura HS33 - Fairly self explanitory, best brake suited for trials period really, I would say very very few people haven't owned some at some point.
Hope XC/Mono - Everyone had a hub that didn't break, and when it did, got replaced for free. And it comes in pretty colours. Again, I imagine most stock riders have used one at one point and I remember thinking they were the best hubs ever, before anyone started to buy Chris Kings. You could always tell a decent trials riders
by the loud clicking from his hub.
Anyway, I'll stop now, post away children.