Lots and lots of cell phone ads, which matches the number of people talking loudly on their cell phones on transit. A certain amount of BANKRUPTCY TRUSTEE! and PHONE DISCONNECTED?!!? ads, but it sounds like not so many as where you live - here transit is the stereotypical domain of not just the poor but also affluent hippies, and there's more money in advertising to the rich environerds and their teenaged/twentysomething progeny. A few years ago the two big universities here started doing mandatory cheap bus passes for all students, which doubled the number of students on transit, so there are a lot of ads targeted to them - like the university bookstores and study-friendly cafés on the bus routes, still more cell phone plans, etc. Ads targetted at the downtrodden: youth crisis lines, fake abortion clinics (pro-life groups who try to talk you out of an abortion), real abortion clinics, mental health hotlines. About 10% of the ad space is taken up by sort-of ads from the transit company, like "We're hiring!" or "Hey, let old ladies sit down, you putz" or "New route starting September 2" or what have you. Some ads for bicycle shops, car-sharing co-ops, and other businesses of interest to the car-free. Ads for events, especially events that are near transit stations.