The website Donkey Kong Hacks has a number of interesting modifications of the original Donkey Kong arcade game. Some of these are intended for use in training, such as Free Run Edition (which removes all the enemies and deadly obstacles) and Skipstart (begins play at maximum difficulty). There are versions that only contain randomly-selected versions of the Girders (a.k.a. Barrels) screen, versions that change the maps, and more. Some, like Donkey Kong Wizardry, change the graphics and change the cutscenes too! The Readme for the Crazy Barrels version explains how to play these hacks in emulators.
There are other fan-made hacks floating around, some available as installable kits from the site DK Remix. Deranged Edition keeps getting harder after difficulty level 5, and Remix and Christmas Remix change the game up a lot, adding alternate maps, bonus stages, and some Rivets stages that fall apart as you remove rivets.