Now that the iPhone 4S has been jailbroken, hacks and tweaks will be coming from every direction. The latest and probably one of the best is OpenSiri.
OpenSiri enables you to launch apps directly through Siri, which was not possible before. However, only the stock Apple apps, Facebook and Twitter will be supported at first. The developer has promised that more apps are on the way.
At the time of writing this OpenSiri is not available on Cydia yet but it should be very soon. So keep checking Cydia regularly to be one of the first to try it.
Please check out AssitantExtensions which is better than OpenSiri!