MarketSci.com has some mechanical trading strategies that are fairly consistent and have a good return. The performance of the strategies is also independently audited by a third-party firm. You can see the results posted on this page. They talk a lot about trading strategies and systems on their blog but don't reveal the actual strategies used by their systems. To follow their strategies, you'll need to have what they call a "managed" account or subscribe to their trading signals.

Covester is a site that'll let you invest in other people's systems in an automated (mechanical) way but their systems may not necessarily be a mechanical trading system. You just end up piggy backing on their trades. The site does give you the benefit of auditing their trades so you can see historically how well they've done. A big negative is that you won't get the details of how each trading system works (or if there is a system at all). Everyone on the site keeps that information to themselves so that they can charge a fee for the trades you make following them.