Per-user voting history in many (most?) states. The fact that a ballot was received from a particular voter (but not what they voted for) is often releasable under public disclosure laws.
Yes, most states track voting participation at the voter level. Some track method of voting, other just are a boolean participation flag for each election.
Individual voting records aren't public in the US (secret ballot). However, whether or not an individual voted in a specific election is often public. (It may vary by state or other jurisdiction.)