Addendum: I should add that some coins do offer a yield, or at least purport to. Dash and Zen being two that come to mind. They offer, or will offer, a yield for running a master node / secure node. When polkadot launches it will offer a yield too.

If we accept airdrops as a form of yield then those would count too; however, I don't know how spendable those airdrops are. If they aren't tradeable then I would value them at zero.