If you are a member of National's Emerald Club, there is no charge for an additional driver(same family, etc). When I rent EC, my hubby is automatically covered. Some of the other companies' frequent renter programs might have similar policies.
It really varies alot and even within a single agency. We usually do National EA so don't worry about it. I would price a couple reservations, then call and ask if it includes extra driver. Sometimes a spouse or business assocatiate is included, sometimes not. Sometimes using a particular credit card might yeild a spouse being covered. I know when I used priceline, even though Alamo for a regular reservation said spouses were covered, for my priceline reservation they were not. I have recently joined all the toher agencies frequent traveller clubs since a) it didn't cost any money,anad b) many include spouses if you're a member.