Built an e-commerce site, pro bono, for an acquaintance and get a profit share each pay period. Occasionally I monitor, upgrade, enhance, and fix it up which results in more income. Great Situation!
Take great care to sign contracts for anything significant you expect to get. I did something like this once and after spending a long time enhancing it, suddenly the acquantance decided he didn't need to share the profit anymore.
Although you'd expect being the dev and possibly admin, you hold some control over the other party?
Better to have more than vague threats on your side, like having the law and vague threats on your side.
I hear that "very specific threats" is also a workable model.