If you are looking to share applications across multiple iPhones, this site gives a step-by-step process on how you can do this. Obviously for free apps, there's not much point to this. But if you have multiple iPhones in your household and paying more than once for the same application offends your sensibilities, this is the site for you.

Supposedly this is even legal. Since the apps fall under the Apple DRM rules, you can legally use the applications on up to iPhones.