Folding bikes have all the efficiencies of normal bikes but they can also be folded and stored in cars, buses, trains and even carried on air flights.
A recent review by Consumer Reports gave top rating to the Brompton C3-E. It was the lightest bike tested and it sells for $675. It is easy to fold, and it folds up so the chain is inside, not on the outside where you can rub against it. In the less expensive range, Consumer Reports named the Dahon Speed D7 a Best Buy. It was the heaviest bike of those reviewed, but it had the lightest price at $380.
One of the least expensive folding bikes is the Citizen 6 Speed bike, which weighs 35 pounds, and is selling for $174 right now. A very similar bike is the 6 Speed Folding Bike, which weighs in at 23 pounds and is selling for $169.
If you are interested in finding reviews and videos of folding bikes, RideThisBike.Com is the best place to look. They recommend the 29lb $650 Montague Paratrooper as a folding mountain bike that can deal with rough surfaces. RideThisBike.Com also has many favorable reviews of the Giatex folding bikes.
Another site, BikeFold, has a huge range of folding bikes for sale, many with price tags over $1000.