RESTSharp and Hammock are client side libraries, used to enhance the capabilities of the HTTP classes in System.Net. In the WCF Rest Starter kit there is a client library called Microsoft.Http that I find very good.
OpenRasta is a server side framework designed to help serve representations to clients. WCF is adequate for REST if you already know what you are doing. It can get painful if you need to step outside its expected boundaries.
Before picking a framework, do spend a reasonable amount of time reading about what REST is. Chances are very good that it is not what you think it is because there is a very large amount of mis-information floating around.
If you look in the Tag Wiki there is a link to the "Implementing REST" wiki on Google code that has this matrix of available frameworks.