Create an online game where most of the game runs on the server.

To pirate your game, players need to create their own server. This means they either have to break into your network and steal your software or create a clone of your whole server software.

Both scenarios aren't that unlikely: They already happened to various companies.

About how to prevent server break-ins, consult http://security.stackexchange.com and http://serverfault.com

When there is a popular online game, there is often a project to clone the server. But such a project usually takes years before it can offer a game experience comparable to yours, so you have plenty of time to attack them legally or put more resources into developing your server than they can put into development of their clone, so you will always be ahead of them.

But please resist the temptation of doing it like a certain publisher cough EA cough and create a game which is actually completely client-sided and single-player, but got a pseudo server-component tagged on just so you can prevent privacy cough Sim City cough.

1. your players will hate you, because it's obvious that the online features are only in your interest and not in theirs
2. your server component won't be doing much, so the amateur coders won't take long to replicate its functionality

Create an online game where most of the game runs on the server.

To pirate your game, players need to create their own server. This means they either have to break into your network and steal your software or create a clone of your whole server software.

Both scenarios aren't that unlikely: They already happened to various companies.

About how to prevent server break-ins, consult http://security.stackexchange.com and http://serverfault.com

When there is a popular online game, there is often a project to clone the server. But such a project usually takes years before it can offer a game experience comparable to yours, so you have plenty of time to attack them legally or put more resources into developing your server than they can put into development of their clone, so you will always be ahead of them.

But please resist the temptation of doing it like a certain publisher cough EA cough and create a game which is actually completely client-sided and single-player, but got a pseudo server-component tagged on just so you can prevent piracy cough Sim City cough.

1. your players will hate you, because it's obvious that the online features are only in your interest and not in theirs
2. your server component won't be doing much, so the amateur coders won't take long to replicate its functionality