I just updated all the travel getaways today on the Travel Deals tab (Living Social, Groupon, Bloomspot, and EverSave), and while doing so I found some great travel certificates for purchase on Groupon. Remember to check out the travel vacation deals as well!
Avis: For $20, you get $40 toward an Avis rental. Certificate expires December 31, 2011 and must be used on a minimum 2-day rental. This certificate is valid for all car classes in Canada and the United States (excluding Alaska). This certificate, however, cannot be combined with another AWD number, must use AWD S666500. The certificate can be redeemed online or over the phone. It is best to check the lowest rate online first prior to purchasing, as this AWD number could potentially be a higher rate.
Frontier: For $25, you get $50 toward any flight from Frontier Airlines. Flight must be purchased by November 16, 2011 for travel prior to February 28, 2011. You may purchase multiple certificates, but are limited to 1 per flight. This is valid for new bookings only and flight must be purchased online. It looks like this IS valid with other offers. Groupon must be purchased by Monday August 1, 2011 at midnight.