People:
* infant
* inmate
* inventor
* immigrant
* interpreter
* instructor
Places:
* island
* inn
* interior
* intersection
Things:
* idea
* issue
* item
* ice
* ink
* iron
* image
* instrument
* industry
* information
* injury
* insect
* invitation
Concepts:
* impact
* importance
* independence
* influence
* inspiration
* interest
Other:
* igloo
* iris
* ivy
This is not an exhaustive list, but it provides a good starting point for common nouns beginning with "i".