Caffeine is an example. Coffee and energy drinks are doing well I'd say.
Kratom, CBD, and delta-8 THC are naturally occurring bioactive substances that are newer to the US market. Both have carved out a pretty nice economic niche with a bunch of claimed health benefits.
A couple of years back, I saw a sign outside a fancy legal highs shop in Fishtown, Philadelphia touting the benefits of kratom as a pre-workout supplement. The insanity that a business was advertising an addictive opioid to healthy, opioid-naive people for better gains in the gym almost makes me want more regulation in this area.
Even synthetic research chemicals are generally legal as long as you add a "not for human consumption" label (and they aren't explicitly banned or analogous to other illegal/regulated drugs).