With Avis (and Budget and Enterprise), your spouse can drive your rental car for free:

The following individuals, if 25 years of age or older and in possession of a valid driver's license, may drive the car with the renter's prior permission:

* Renter's employer (while on company business);
* Renter's fellow employee (while on company business
* Renter's spouse or life partner.

The above additional drivers are considered "authorized drivers" and need not be present at the time of rental, nor do they need to sign an additional driver form.

(At Hertz, the same is true for #1 Club Gold members.)

The above is for US.