Common words:
* ignorant: lacking knowledge or information
* ignore: to refuse to pay attention to
* ignite: to set on fire; to cause to start burning
* imagine: to form a mental picture or idea
* imitation: the act of copying something
* impatient: easily annoyed by delays or waiting
* impossible: not able to be done or achieved
* impress: to make a strong or favorable impression on
* improve: to make something better
* inquire: to ask for information
* install: to set up or fix something in place
* instant: happening immediately
* intense: very strong or extreme
* interest: a feeling of wanting to know or learn more about something
* interrupt: to break into or stop something
* introduce: to present someone or something for the first time
Less common words:
* ignoble: not honorable; mean or base
* ignominy: public shame or disgrace
* ignoble: of low birth or character
* igneous: relating to or formed by fire
* ignis fatuus: a misleading or deceptive light
* igloo: a dome-shaped house made of snow
* iguanodon: a large, plant-eating dinosaur
* iguanas: large, herbivorous lizards
* iguanine: a type of lizard
* ignavia: cowardice or laziness
This is not an exhaustive list, but it provides a good starting point for words that start with the prefix "ig."