Also take a look at ILSpy by SharpDevelop. It’s in early stages of development and they just made a release on the 24th of February. That in itself works pretty good for me. From their website:
ILSpy is the open-source .NET assembly
browser and decompiler.Development started after Red Gate
announced that the free version of
.NET Reflector would cease to exist by
end of February 2011.
Update: JetBrains has released dotPeek, its free .NET decompiler.
Update 2: Telerik also has a free decompiler: JustDecompile.