[zaphar]

     "everybody agree on a standard prefix for basic things 
     like calculators" HAHAHAHAHAHAHAHAHAHAHA

That's a URN. Unfortunately they never really got adopted. But thy solve this exact problem.

urn:calc:rpn

You click the link and your mobile phone then looks to see if any of your calculator apps satisfy that urn.

If they find any they open it up or present the user with a choice between them or open the one the user configured as their choice.

If they don't find any they check the app store or web and offer the user the opportunity to install one that satisfies that urn.

Actually this is exactly what androids intents do as well minus the app search and install. But I don't think they use urns under the hood.

[shortstuffsushi]

I was actually going to reply something along the same lines. These sorts of services exist on both iOS and Android (can't speak to Windows Phone). Not for the provided example, but for Android, the Intents like you mentioned, and iOS has a similar concept for things like "Apps that are capable of routing/mapping" and possibly other things. Neither of these are done through the example "standard prefix," but both are through by adding some standard permission or key to your app.