The list is long! Just show the European Youth Card and you have special discount on products and services in areas such as:

Transport: urban / interurban transports, railway tickets, ferry tickets.
Education: Greek universities education distance programs, centers of Post-secondary Education, Vocational Training Institutes (VTI), Vocational Training Centres (VTC), language schools, dance classes.
Tourism: travel agencies, hotels, rooms for rent, camping, hostels, car rentals.
Culture: cinemas, theaters, museums, cultural events, concerts.
Stores: clothing stores, optical, hairdressers, jewelers, bookshops, bicycles, household items / gifts.
Entertainment / Dining: Restaurants, bars, cafes, clubs.
Health: clinics, diagnostic centers, doctors, pharmacies.
Sports: gyms.
Beauty: hair salons, beauty salons.
Services: telephony and internet.

You can see a list of all the firms in Greece here!
You can find discounts abroad by selecting a country here!