'Smart Grid' can mean something different to everyone, which is why the responses include mostly programming environments, not plug and play solutions. Depending on your specific interest, you will likely have to build your own model. While some programming environments may have prefabricated modules that simulate certain power sector features, they may require financial and time investment and may not be very useful for other parts of your work. If you already have access to one such as those listed by Abhisek, then great. Otherwise, pick a tool you can use elsewhere, such as matlab, octave or *gasp* excel.
The best way is programming the mathematical models of each agent of Smart Grid (such as WPP, PV, Battery, PHEV, network, etc.) in MATLAB. Moreover, if you like to determine the optimal size and capacity of each kind of small-scale renewable resources economically, HOMER will also be a good choice.