They typically have 1 major event with 1000 or so free tickets per year. This past season it was one of the non-conference games and it ended up raining pretty bad. The year before, it was MSU on Veteran's day and that was amazing. Not sure what they'll do this year. This is the first time I've seen spring game tickets on there in bulk.
Beyond that, most home games have individual donors that will give out 2 or 4 tickets at a time, but those are really hard to get (I've never won a lottery for one). I've been seeing a lot of women's basketball and men's/women's hockey this year, they usually have unclaimed tickets for those. I don't follow other sports so not sure if there are others they give away.