Technically, Snape did do Voldemort's bidding, but he typically did it in ways that were subtly (or not-so-subtly) undermining to Voldemort's purposes, such as
the fact that he planted the idea for Harry Potter's transfer on the Order.
Technically, he gave Voldemort correct information (such as the real date of the event), which was enough to divert suspicion from him, but obviously his information was incomplete to say the least.
He was also able to protect the students at Hogwarts to some degree (e.g. witness the fact that he sent Ginny Weasley to do detention with Hagrid in the Forbidden Forest instead of being subjected to the Cruciatus Curse).
Perhaps the most significant example, of course, was
the fact that he smuggled the Sword of Gryffindor to Harry Potter so they could destroy the Locket horcrux with it.
Dumbledore also implied in the 5th book that Snape gave him some rather critical information. It's not specified exactly what that was, but
presumably it included (among other things) the fact that the Potters were being targeted by Voldemort
Also closely related: What concrete aid did Severus Snape provide the Order while he was under cover?
It's also worth noting that, due to the fact that the Trio was traveling with a portrait of Phineas Nigellus Black, Snape was able to pass them information and keep tabs on them at the same time. In fact, there were times where he appeared to have known more about Harry Potter's whereabouts and activities than the Order itself.