There are different types of simulation products, but 3 of them are the best currently available.
There are two main Poker simulation programs that allow you to play against computer opponents or other people using the software over the internet. The programs are DD Poker and Poker Academy Pro.
They allow you to get realistic playing experience without having to risk any money. DD Poker recently became a completely free product, but Poker Academy Pro has a one time license fee.
A number of simulation programs allow you to analyze poker hands in great detail but the most widely used is a free product is PokerStove. It allow you to set up poker hands with a variety of options such as choosing hand ranges and flop types of hands in poker and calculates different probabilities and hand equities by simulating thousands of hands of poker in a very short amount of time.
The detailed information is useful when analyzing poker hands and working out what kinds of hands you should be going all in with against different opponents.