AKA the pink sheets. Many companies start there - most fail, but occasionally one rises to be listed on a major exchange. Most pink sheet companies are extremely tiny and the IPO was self-underwritten. In addition, it is sometimes hard to buy the stock because they'll have a limited number of market makers.
OTOH, if you are willing to REALLY do the research, you can get lucky and find a spectacular winner from time to time. In the mid-80's, I bought 35,000 shares of a dinky company called Morgro chemical at a whopping three cents per share. Sold it 18 months later at a net (after commissions front and back) of about 90 cents a share.🙂
Russ, NCNE