Waiters/waitresses first and foremost. I won't eat out if I can't tip generously and TBH I kinda look down on people who do. :/
I always tip people who deliver things to me. UPS is super sneaky so I can't offer them anything. A lot of our local stores (like walmart where I work) require employees to decline any gifts from customers, so I've gotten in the habit of not tipping the people who bag my groceries, and I am young and able-bodied and can carry my own stuff out of the store.
In Mexico, tipping is... socially mandatory, really. If you appreciate some service that is provided, it is acceptable and appreciated if you tip. I always left tips for the fishermen's wives who ran a stand by the beach and made the most awesome fish tacos. We had a language barrier issue, but I showed my appreciation with tips and they showed their appreciation by always tucking a little something extra into my bag. That's how you build a relationship with businesses in mexico, since most places are small and privately owned. If you're a stingy dick, they're not gonna go out of their way to offer you service next time. Especially food and drink service. Some people just got blown off entirely by the entire waitstaff of a restaurant I liked because they didn't "believe" in tipping. Um, hello? It costs like ten bucks for a giant shrimp dinner and you're thinking the restaurant is making bank off that? LOL.
Sometimes I'm just flat broke. Thats when I buy ramen and eat at home, and don't order stuff that has to be delivered or go out to a sit-down restaurant and eat.
But yeah. I try to tip especially well when I know the worker is in a tip-dependent job, not getting paid minimum wage.