Pokéstops are a source for a great variety of items, especially Pokéballs, potions, revives and berries. The quality of the received items depends on the trainer level (see Table 1). Pokéstops also reward rare evolution items that are needed to evolve certain Pokémon (e.g. Upgrade (Porygon), metal coat (Scyther, Onix), …). Successfully spinning a Pokéstop every day for 7 days grants bonus items and a guaranteed evolution item. Spinning 10 unique Pokéstops within 30 min also grants bonus items and experience. The item drop rates have been changing in the past, but at the moment normal Pokéstops provide mostly Pokéballs, while gym Pokéstops also provide potions and revives at a higher rate.
Table 1: Trainer level and unlocked items (Source)