Voldemort didn't return and summon the Death Eaters until after the events of "The Goblet of Fire." Prior to this, Snape would have only been observing Slytherin house and, potentially, any stray Death Eaters who would associate with him after Voldemort fell in Godric's hallow. Dumbledore was suspicious that the Dark Lord had not been fully destroyed, so Snape's role during this time was to watch for signs of his return and monitor the remnants of the Death eaters.
After Voldemort's return, one sure piece of information which passed from Snape to Dumbledore was the plot to have a student kill Dumbledore. Information on plots and the Death Eaters in attendance would surely be gained from the Death eater meetings which Snape attended; However, the books do not specifically mention this information being passed down.