The top 5 IL viewers / disassemblers are:
If you have Visual Studio or the Windows SDK, you already have ildasm.exe. It's the offical IL disassembler from Microsoft.
ildasm is an IL viewer only.
Download from www.devextras.com/decompiler/
CodeReflect is an IL viewer and .NET decompiler. You can choose to view either IL or generated source code.
Download from www.telerik.com/products/decompiler.aspx
JustDecompile is an IL viewer and .NET decompiler. You can choose to view either IL or generated source code.
Download from www.red-gate.com/products/dotnet-development/reflector/
Reflector is an IL viewer and .NET decompiler. You can choose to view either IL or generated source code.