National Rental Car Emerald Club members - spouse is free on the rental car. Check out www.mousesavers.com for directions how to get a free Emerald Club membership.
Going from memory, so I may be wrong, but I think that that's one advantage of joining the "clubs" Emerald aisle and Fastlane both get you your spouse for free. I wouldn't be surprised if most of the other one's do too. Years ago, when we used alamo, spouse was free UNLESS it was a priceline reservation - go figure! Anyway, I would call and check after you have your reservation if you're unsure. It could be that some rates allow it and some do not.