Here are several discounts for Visa and Mastercard holders that are valid until 10/31/09 Visa and 12/31/09 Mastercard. These are especially handy as many of the general coupons have not been extended yet. Requires use of said card to pay for rental:

Visa:

Visa Car Rental discount codes for Alamo:
Visa holders can receive up to 20% off their next rental with Alamo by using Alamo contract ID # 7014615.

Get a free day on your next 5 day car rental by using Alamo coupon code AF3475JDG. Expires 10-31-09.

Get $25 off your next 5 day plus rental of a convertible, SUV or Minivan by using Alamo coupon code AD63514DS. Expires 10-31-09.

Visa Car Rental discount codes for Hertz:
Visa holders receive $5 off per day on their next rental with Hertz up to $15 off. Use Hertz CPD # 1244810 and Hertz Promotion Code # 111311 for weekly and Hertz Promotion Code # 119766 for weekends.

Visa Car Rental discount codes for National:
Visa holders can receive up to 20% off their next rental with National by using National contract ID # 5027112.

Get a free day on your next 3-5 day car rental by using National coupon code NF4182JDK. Expires 10-31-09.

Free upgrade at National in US and Canada. Valid when reserving a compact through fullsize by using National coupon code NU22872JDY. Expires 10-31-09.

Visa Car Rental discount codes for Avis:
Visa holders get a free day on their next 5 day rental of a midsize thru fullsize with Saturday night keep from Avis. Use Avis coupon code TUCA044, expires 10-31-09.

Visa Car Rental discount codes for Budget:
Visa holders get a free upgrade on their next rental of a midsize thru fullsize from Budget. Use Budget coupon code UUCZ010, expires 12-31-09.

Found at http://www.carrentalsavers.com/visa

Mastercard:

Mastercard Car Rental discount codes for Alamo:
Mastercard holders can receive up to additional 20% off their next rental with Alamo by using Alamo contract ID # 7013993.

Get a free day on your next 5 day car rental by using Alamo coupon code AF2604JDN. Expires 12-31-09.

Free upgrade at Alamo in US and Canada. Valid when reserving a compact through midsize by using Alamo coupon code AU1787JDK. Expires 12-31-09.

Free upgrade at Alamo in Europe, Middle East and Africa. Valid when reserving a compact through midsize by using Alamo coupon code AU1Z64JDW. Expires 12-31-09.

Mastercard Car Rental discount codes for National:
Mastercard holders can receive up to additional 20% off their next rental with National by using National contract ID # 5026890.

Get a free day on your next 5 day car rental by using National coupon code NF2583JDY. Expires 12-31-09.

Free upgrade at National in US and Canada. Valid when reserving a compact through fullsize by using National coupon code NU22106JDN. Expires 12-31-09.

Free upgrade at National in Europe, Middle East and Africa. Valid when reserving a compact through fullsize by using National coupon code ND1468DDA. Expires 12-31-09.

$15 off weekly at National worldwide. Use National coupon code ND1468DDA. Expires 12-31-09.

Mastercard Car Rental discount codes for Dollar using Mastercard:
Mastercard holders get 5% off their next rental with Dollar in US and Canada by using Dollar corporate code BR0312. Waives 21-24 underage fee.

Found at http://www.carrentalsavers.com/mastercard

If you haven't already, you might want to add this valuable information to the stickied rental car discounts thread (at top of travel deals forum).

A quick question here. If I use the coupon code of VISA, however pay by my master card, will it be valid?
If not, I may need to find others who hold the VISA card.