Very briefly, the Glagolitic alphabet. Our oldest Old East Slavic manuscripts, the Ostromir Gospels (the oldest dated ms, 1056/57) and the Novgorod Codex (the oldest ms we have, written some time in the late tenth or early eleventh centuries) are both in Cyrillic. To my knowledge, Glagolitic was already falling out of use among South Slavs by this time. I don't believe we have any Old East Slavic manuscripts written in Glagolitic, and what does survive is a handful of inscriptions.

There's some speculation about a pre-Christian writing system called "strokes and incisions" (čerty i rezy) in a handful of medieval sources, but all the putative examples of it have turned out to be hoaxes.