Petsafe wireless dog fence.   This is the one that most people really need. 

My wife and I have had dogs all our lives.  We have raised show dogs, and had many dogs as pets.  Over the years we have had stake tie outs, overhead wire 'trolley' systems, even a simple rope tied to the porch, all in an effort to keep our dogs in our unfenced yard.  We finally broke down and put in the 'underground wireless' system (I won't name it here) in hopes of being rid of the tie outs. What a mistake!  Our dogs would take off after a rabbit or other chaseable thing and run right over the fence.  By the time it shocked them once, they were across the fence and would not come back into our yard because they would get shocked.  It actually kept them out of our yard!

Petsafe works in a different way.  As long as the collar is recieving the signal it won't shock.  So when the dog leaves the yard, it shocks them continually (up to 30 seconds) until they return into your yard.  No flags needed.  No training needed.  Litterally after 1 'crossing' my dog learned.  For the next couple of days she didn't stray far from the house.  After she 'crossed' about 3-4 times, she knew how far she could go and would not leave the yard. 

If you have shorter dogs - less than 6" tall - an inground wireless system will probably work fine for you.  If you have a dog taller than about 6"  - Spend the money and get Petsafe wireless dog fence.  It will save you from freezing in the winter, loosing your A/C in summer and loosing your dog year round.