Free online ticketing and event management solution.
For years, I have harped on clients to aggressively sell advance tickets to shows. PayPal makes it easy to collect a few bucks for low (or no) fee, but one of my clients mentioned that it was still tough to keep track of who has paid for multiple shows.
Enter MollyGuard. Can't believe I forgot about these guys. I used their event registration service a while ago for some live events. Since then, the founders have done so well that they've eliminated the fees on their event management software. Amazing. And so simple.