There are many Win32 API calls / libraries like kernel32 or shell32 that may be usefull to call from C#. A common method of using them is looking up the method definition on the pinvoke.net website, and using the dllimport definition from there.
This process is required for every method you wish to use.
Are there assemblies availble that contain common / most [DllImport]ed API functions?
I'm looking for something i can use directly from C# (as a reference), so i don't have to gather and combine all kinds of DLLImport statements myself (with risk of errors).
That I know of, there is no widely known bundle of code wrapping the Windows API for .NET; I could see it being useful in certain circumstances, however, in most cases, if you need to use so much of the native Windows API then you should maybe ask yourself whether or not you're utilising the .NET framework properly and/or why use .NET at all?
Regardless of the above opinion, you can find a pretty comprehensive source of the definitions at http://pinvoke.net/.