Having spent some time with algorithmic trading platforms, I find Quantopian to be one of the most powerful and flexible in its offerings as follows:
1. Free to sign up, backtest strategies and compete in their trading contest
2. Free plan offers many powerful data feed which are other subscription only
3. Platform is built around a large and growing community of active users
4. Some tools are open-source, and public algorithms can be cloned for use
That said, one of the simpler strategies is a portfolio rebalance across various asset classes. Turns out it is also conservative as well and intended for retirement accounts.
The strategy and algorithm are outlined in this community discussion post; the algorithm can be cloned as follows:
1. Sign up for a free account
2. Clone algorithm and run a backtest over a given time period
3. Once algorithm runs a backtest without errors, then deploy it live
Moving forward, feel free to modify the algorithm, which will then be under the code section of the account.