TOS platform on TDAmeritrade is excellent. They execute your demo order at the mid point of the bid ask option price and that is a fair generally obtainable price so your trading results will be representative of a live market trade.
My dummy broker account is powered by TradeIT. It also acts as a broker aggregator that integrates top brokerages so I can trade in using a configurable dashboard in one place!
As @MattZ pointed out above, the free demo version of the TT platform includes our full suite of options trading tools. https://optimusfutures.com/TradingTechnologies-Platform-Demo.php
You can sign up for a demo account of CityTrader at www.optionscity.com/citytraderdemo. CityTrader is web-based platform for trading both futures and options. They also have a free accompanying mobile app available on iTunes app store and Google Android.