Cost-Effective Methods For Implementing Two Factor Authentication

Typically, implementing two factor authentication means buying tokens or smart cards for all of your users. This can be expensive (from what I have seen $50/user is typical in smaller deployments) and it is only manageable for users with whom you have an existing relationship (employees, in some cases customers). So how do you implement two factor authentication for "cheap"? Here are a few methods I have seen used...

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