PC Review has compiled a list of Siri commands, you can read it over here.
PC Review has a comprehensive article on how to get the most out of Siri on the iPhone 4S. It includes usage examples for all compatible apps as well as some tips you may not have known:
"There are a huge range of tasks that Siri can help with, and although it is still in beta (as of January 2012) Siri could change how we use our mobile devices forever.
Whilst we are all used to speaking to other people on the phone, having a conversation with your phone can be an alien concept. The attitude that a lot of people have is “why use Siri when you can simply type in the information / query etc yourself?”. Hopefully this guide will inspire and encourage a change in the way you use your iPhone 4S."