The following is the best discussion on addons I remember seeing:

https://www.dual-boxing.com/threads/...ighlight=addon

People use lots of different ones. The key ones are:

ISBoxer (note this one should be added automatically for you by isboxer);
EMA-Classic
DynamicBoxer
WowLua (for development/debugging)

Most people like some soft of healing addon like Clique, VuhDo, Healbot, etc.

Some folk like replace the standard UI with ELVui or others like that.