I have just written an explaination of the $500 Free Play + 100% match to $100 at Jackpot City in our slots forum. The basic premise is that players get $500 of free play for 30 minutes and can then move any winnings (anything over and above the $500) accross to their real money account.
If the player does decide to make a real money deposit, they will also be eligable for a 100% match bonus meaning when they deposit $100 they will have $200 to play with (plus any winnings they transferred accross from the free play). This seems like a great risk free way to sample the casino to me.
In addition I have found out that as this is part of the same group of casinos as Lucky Nugget (the Belle Rock group), Jackpot City is also offering free to enter slots tournaments every 2 hours with $75 in prizes per tournament.