Groupon is offering a $40 voucher towards a car rental from Avis Rental for $20. This voucher is valid at participating locations in the United States, Canada, Puerto Rico, and the US Virgin Islands (Alaska excluded). Here is a list of non-participating locations.
While $20 off a rental is great, there are some limitations. You must make your reservation online at least 24 hours in advance and there is a 2-day rental minimum. The big exclusion is that this voucher cannot be combined with any other discount or coupon. Many times a AAA, Costco, corporate discount, etc. Can save you more than $20 on your rental.
You do, however, have more than a year to complete your reservations as the voucher expires 12/31/14 (rental must be completed by then.
While I am not sure if I would speculatively purchase this voucher, if you have an upcoming rental need, see if this $20 off essentially is better than any other discount you’d receive.