The most common is the A-Frame, but be careful as AFAIK they are now illegal to use to tow in the UK as they are not braked.

We had an A-Frame like this and it worked fantastic, only downside was having to slide under the car to attach.

http://www.cjautosheywood.co.uk/orig/aframe.jpg

However a far better solution is a unit like this, the AA/RAC use them, they are braked, perfectly legal and you almost forget the car is behind you.

http://www.ashaux.com/Pictures/Trailer/Photo05.jpg

You can get a simpler turntable style dolly but the one above allows for fantastic maneuverability