The online version is free, but it can't do as much as the pay version. But you get what you pay for...
You can do quite a bit of backtesting there just by signing up. You can try other systems or write your own and test them for free on the site. It's limited compared to purchasing the s/w, but it's doable.
I hope moderators wont delete this message. Here is the link: http://www.metaquotes.net/files/mt4setup.exe Best Regards