The ASP.NET cache object is located in the System.Web namespace, and because it is a generic cache implementation, it can be used in any application that references this namespace.

The System.Web.Caching.Cache class is a cache of objects. It is accessed either through the static property System.Web.HttpRuntime.Cache or through the helper instance properties System.Web.UI.Page and System.Web.HttpContext.Cache. It is therefore available outside the context of a request. There is only one instance of this object throughout an entire application domain, so the HttpRuntime.Cache object can exist in each application domain outside of Aspnet_wp.exe.

The following code shows how you can access the ASP.NET cache object from a generic application.

HttpRuntime httpRT = new HttpRuntime();
Cache cache = HttpRuntime.Cache;

After you access the cache object for the current request, you can use its members in the usual way.

Source: Caching Architecture Guide for .NET Framework Applications

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnbda/html/CachingArchch2.asp

The statement "safe for multithreaded operations" can be misleading.
The type that you put in the Cache must implement it's own thread synchronization mechanisms for updating.
Conclusion: Only read from the object in Cache [as an rvalue]. Never update unless the object in Cache is thread-safe.