Red Coin (Replaces Red keycard) Does nothing, but can be used for coin collecting gimmicks like what was done in Mayhem17 map15.

Bowser (Thing ID 72) triggers KeenDie on death, which means when all Bowsers on a map are killed sector tag 666 opens.

Lava Lotus (Thing ID 6) Blasts the player upward with an Archvile blast for 20 damage. Can be used to get around or as a hazard.

Poison Mushroom (Thing ID 84) Looks like a normal Mushroom powerup until you get close, but then transforms and does damage if you don't react quickly.

Mario 2 grass (Thing ID 5) Just here to have another animated decoration if you want.

Donut lift (Thing ID 23) A decoration which is sunken into the ground. You can use this on invisible bridges.

Virtual Boy (Graphic swap of Liteamp) Technically does nothing by itself, but a custom colormap (VBOYCMAP) is included in the addon WAD which makes everything at 
255 brightness use red and black colors in sectors it is applied to. Requires experimentation to make work

Non-flying cacodemon (Thing ID 89) originally used for a pirahna plant pipe gimmick.

Damager (Thing ID 12) Causes constant melee damage, used for death zones.