AirTran
Type of Voucher(s): Round-trip ticket
Any blackout dates? Yes
How do you redeem? Online or by phone
Can you split the amount? Not applicable
The expiration date is... Fly-by
Will the airline honor/replace lost vouchers? Yes

American Airlines
Type of Voucher(s): Credit toward a flight
Any blackout dates? No
How do you redeem? By phone or at the airport
Can you split the amount? Yes
The expiration date is... Book-by
Will the airline honor/replace lost vouchers? No

ATA Airlines
Type of Voucher(s): Round-trip ticket or credit toward a flight
Any blackout dates? No
How do you redeem? By phone, by mail, or at the airport
Can you split the amount? No
The expiration date is... Book-by
Will the airline honor/replace lost vouchers? No

Continental Airlines
Type of Voucher(s): Credit toward a flight
Any blackout dates? No
How do you redeem? Online, by phone, or at the airport
Can you split the amount? Yes
The expiration date is... Book-by
Will the airline honor/replace lost vouchers? Yes

Delta
Type of Voucher(s): Credit toward a flight
Any blackout dates? No
How do you redeem? Online, by phone, or at the airport
Can you split the amount? Yes
The expiration date is... Book-by
Will the airline honor/replace lost vouchers? Yes

JetBlue Airways
Type of Voucher(s): Two one-way tickets or credit toward a flight
Any blackout dates? No
How do you redeem? Online or by phone
Can you split the amount? Yes
The expiration date is... Book-by
Will the airline honor/replace lost vouchers? Yes

Northwest Airlines
Type of Voucher(s): Round-trip ticket or credit toward a flight
Any blackout dates? No
How do you redeem? Online or by phone
Can you split the amount? Yes
The expiration date is... Fly-by
Will the airline honor/replace lost vouchers? Yes

Southwest
Type of Voucher(s): Credit toward a flight
Any blackout dates? No
How do you redeem? Online, by phone, by mail, or at airport
Can you split the amount? Yes
The expiration date is... Fly-by
Will the airline honor/replace lost vouchers? No

United
Type of Voucher(s): Round-trip ticket or credit toward a flight
Any blackout dates? No
How do you redeem? By mail or at the airport
Can you split the amount? No
The expiration date is... Book-by
Will the airline honor/replace lost vouchers? No

US Airways
Type of Voucher(s): Round-trip ticket or credit toward a flight
Any blackout dates? No
How do you redeem? By phone, by mail, or at the airport
Can you split the amount? No
The expiration date is... Book-by
Will the airline honor/replace lost vouchers? Yes, but only if you have an e-voucher

This pertains to vouchers handed out for domestic flights. All vouchers are redeemable for domestic travel and expire after a year.

Only AirTran and JetBlue won't let you use a voucher for a friend.