You can use ILDASM (Intermediate language disassembler).
Other tools like .NET Reflector, dotPeek, JustDecompile or ILSpy can also show the IL code, in addition to showing it in C#, Visual Basic .NET and/or other languages.
LINQPad can also show the IL of your code snippets, but not the IL of arbitrary assemblies.
EDIT: actually dotPeek only shows C# code, not IL