Ketene cycloaddition reactivity
Ketenes can participate in [2+2] cycloadditions with their $\ce{C=O}$ double bond.  Ketene dimerization is the best known example of such a cycloaddition.



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As you add substituents to the ketene $\ce{C=C}$ double bond, you begin to favor cycloadditon across the carbon-carbon double bond at the expense of cycloaddition across the carbonyl double bond.

Some Diels-Alder cycloadditions also involve adding across the ketene carbonyl.



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With both the [2+2] and [4+2] cycloadditions often the kinetic product involves cycloaddition across the carbonyl, but on mild heating this cycloadduct can rearrange to the product from cycloaddition across the carbon-carbon double bond, the thermodynamic product.

A very nice review of the synthetic and mechanistic aspects of ketene cycloadditions can be found here.