This WoW addon makes assisting easy whether you're assisting or want to be assisted, by allowing you to set a keybinding which will automatically target the tank's target or put a raid mark on your current target if that's you. A second keybinding allows you to manually set the player you want to assist or whether you want to mark. It is meant to be a minimal-fuss assist addon, and is particularly good for people who both tank and DPS and don't want to maintain separate keybindings for these situations.
This is a button you can mash to make dungeons go more smoothly. Though make sure you know where/what your target is before you attack it; once in Naxxramas I pulled an extra group of trash because I assisted a tank who was tab-targeting through it.
A configuration panel allows you to set the keybindings (which are also found in the blizzard keybindings interface), to change the mark that you apply on the current target when you are tank/assist (the default is a skull), and to choose if you'd like the assist key to automatically apply marks when there is no tank or person to assist. If you have a LibDataBroker display addon, AssistAssist provides a plugin which lets you see what it is currently set to do, click to manually set, and right click to open preferences.
If you are in a raid or other situation where there are multiple tanks, it will automatically mark if you are a tank and assist the person who most recently became a tank otherwise. This will only possibly be what you want it to do; this is the primary reason the manual set assist button exists.
If the tank/person to assist changes in combat the change will not take effect until you leave combat (this is a restriction Blizzard has created in their API to prevent automation.)