We have a detailed explanation on our blog. The short version comes down to this:
Sponsorship, partnership, and licensing are all viable options. So are data aggregation and ads, but we'd like to stay away from those. There's a long lead time on most of these options. Donations need to keep us afloat until then.
Warhorn has over 20,000 active users per month. If just 3% of you donate $1/month or $10/year we can cover our operating costs.
Warhorn started out as simple app that Brian made for himself to manage a Living Greyhawk convention. It's remained a labor of love for almost fifteen years. You, our supporters, are the only reason that Warhorn has come this far. Thank you.