How long do you want to simulate? Most brokers have a 2 weeks trial period, so just sign up with them. Also, what do you want to trade? If futures, here is the T4 platform:
NinjaTrader has a built in simulator that will make up prices, but it also have Market Replay where you play back real data from prior days. Both of these options are completely free.
You can also go with something like MultiCharts where like NinjaTrader you can plugin to a bunch of different demos if you only need it for a short period.